BUG FIX: wide_to_long modifies stubnames
Simple fix that prevents modification of the list `stubnames`, which is an argument to `wide_to_long`. 

Closes #9204

This needs a test and an addition to the bug fixes section of what's new for 0.16.
 I have never written a test. I see you have a test written already for wide_to_long:

``` python
class TestWideToLong(tm.TestCase):
    def test_simple(self):
        np.random.seed(123)
        x = np.random.randn(3)
        df = pd.DataFrame({"A1970" : {0 : "a", 1 : "b", 2 : "c"},
                           "A1980" : {0 : "d", 1 : "e", 2 : "f"},
                           "B1970" : {0 : 2.5, 1 : 1.2, 2 : .7},
                           "B1980" : {0 : 3.2, 1 : 1.3, 2 : .1},
                           "X"     : dict(zip(range(3), x))
                          })
        df["id"] = df.index
        exp_data = {"X" : x.tolist() + x.tolist(),
                    "A" : ['a', 'b', 'c', 'd', 'e', 'f'],
                    "B" : [2.5, 1.2, 0.7, 3.2, 1.3, 0.1],
                    "year" : [1970, 1970, 1970, 1980, 1980, 1980],
                    "id" : [0, 1, 2, 0, 1, 2]}
        exp_frame = DataFrame(exp_data)
        exp_frame = exp_frame.set_index(['id', 'year'])[["X", "A", "B"]]
        long_frame = wide_to_long(df, ["A", "B"], i="id", j="year")
        tm.assert_frame_equal(long_frame, exp_frame)
```

Could I just build an assertion within this existing test where I check the stubnames before and after the function call?

And how do I add to the bug fixees section? I do not think I saw in the developer's guide for contributing to pandas.
 @jtorcasso ideally just add another tests just like this one, but that asserts the stub names don't change (you could modify this one as well, whatever works). Then add in doc/source/whatsnew/0.16.0.txt a note
 @jtorcasso to be concrete, you can add another `test_...` function (like `test_simple`) to the `TestWideToLong` class. 
You can eg just use the example you showed in the issue (#9204) that failed, asserting that the stubs list in the end is still the same.
 Hopefully this last commit did the job.
 merged via https://github.com/pydata/pandas/commit/fda50121453f76142b00ff57b017b8a3ef692f69

thanks!
