BUG/TST: reset setitem_copy on object enlargement
related #5581
closes #5597

@jtratner, cc @TomAugspurger 

this fixes the warning in `tests/test_index.py`. I don't believe this changes any thing else.

can you give a whirl.
 Yep that did it.
 @TomAugspurger pls give this updated one a whirl... can you post a script of what you are doing...I can add as tests...
 I'm just running the `index.py` test.

On master:

```
(pandas-dev) ~/E/p/l/p/s/p/p/tests (master↑ ) pwd
/Users/tom/Envs/pandas-dev/lib/python2.7/site-packages/pandas/pandas/tests
(pandas-dev) ~/E/p/l/p/s/p/p/tests (master↑ ) nosetests test_index.py
................................................................................................................................................................./Users/tom/Envs/pandas-dev/lib/python2.7/site-packages/pandas/pandas/core/generic.py:1029: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
  warnings.warn(t, SettingWithCopyWarning)
.................
----------------------------------------------------------------------
Ran 178 tests in 1.131s

OK
```

This PR:

```
(pandas-dev) ~/E/p/l/p/s/p/p/tests (jreback-copy_warn=) nosetests test_index.py
..................................................................................................................................................................................
----------------------------------------------------------------------
Ran 178 tests in 0.928s

OK
```

The causing the warning was this one:

``` python
    def test_set_value_keeps_names(self):
        # motivating example from #3742
        lev1 = ['hans', 'hans', 'hans', 'grethe', 'grethe', 'grethe']
        lev2 = ['1', '2', '3'] * 2
        idx = pd.MultiIndex.from_arrays(
            [lev1, lev2],
            names=['Name', 'Number'])
        df = pd.DataFrame(
            np.random.randn(6, 4),
            columns=['one', 'two', 'three', 'four'],
            index=idx)
        df = df.sortlevel()
        self.assertEqual(df.index.names, ('Name', 'Number'))
        df = df.set_value(('grethe', '4'), 'one', 99.34)  # <<<< this line
        self.assertEqual(df.index.names, ('Name', 'Number'))
```
 Why don't we default SettingWithCopy to error in the test suite? (and then
make sure the default is warning and test what happens with warning/None)
 @TomAugspurger ok...that was my test case as well

@jtratner in theory you could do that after this PR. In `test/test_indexing.py`, the options are set (for testing), but other than that the default ('warn') is used.

Let me change it
 @jtratner I can set it in a couple of the likely test suits...any way to change it 'globally' though? (only for testing)?
 the _right_ way to do this is to have a pandas unittest class then have all tests inherit from this, and have the setup in a sub always call this, but more work that what I am doing
 too hacky?

I don't want to just import the test suite to actually trigger this so I think have to actually call it at the top of each file (because you could just call that file independently). We don't have a 'global' test suite setup ATM
 Hmm, that's true.
 Why not just define a class with a setUp method and have other modules
inherit that?
 @jtratner you could do that

but then you have to make sure that an existing `setUp` calls it and change the inheriting from unittest every where

that _would_ then allow say ignoring of `FutureWarning` and such in an easy way (but so will this)
 Okay, however you want to do it is fine with me. Maybe there's a global
nose setup that you can do via environment variable?
On Nov 25, 2013 8:54 AM, "jreback" notifications@github.com wrote:

> @jtratner https://github.com/jtratner you could do that
> 
> but then you have to make sure that an existing setUp calls it and change
> the inheriting from unittest every where
> 
> that _would_ then allow say ignoring of FutureWarning and such in an easy
> way (but so will this)
> 
> —
> Reply to this email directly or view it on GitHubhttps://github.com/pydata/pandas/pull/5584#issuecomment-29218769
> .
 looks like you can do `setUpClass` a class method (assuming  you sub-class).... then wouldn't need to have the `setUp` actually call `super`...
 @jtratner alright...revised to baseclass all TestCases....whoosh!
 Okay I'll take a look tonight.
 @jtratner give this  a look see pls
 @jtratner ok with this?
 @jreback this looks fine, though I realized we could have just defined a single `setup_package` and `teardown_package` in each of the testing modules instead:

So  could have just done this:

``` python
def setup_package():
    pd.set_option('chained_assignment','raise')
    #print("setting up: {0}".format(cls))

def teardown_package():
    #print("tearing down up: {0}".format(cls))
    pass
```

and then in test `__init__.py` files just do

``` python
from pandas.util.testing import setup_package, teardown_package
```

so knowledge for the future.
 ok that's prob about the same amount of code I guess
good 2 know though
