BUG: assignment to multiple columns when some column do not exist
- [x] closes #13658
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Previous PR: #26534

In particular, the following code now behaves correctly.

```python
import pandas as pd
df = pd.DataFrame({'a': [0, 1, 2], 'b': [3, 4, 5]})
df[['a', 'c']] = 1
print(df)
```
v0.22: error

master: column `'c'` is converted to index `-1`, which causes last column to be overwritten.
```
   a  b
0  1  1
1  1  1
2  1  1
```

After this PR:
```
   a  b  c
0  1  3  1
1  1  4  1
2  1  5  1
```
@jreback any more changes required? pls rebase as well @howsiwei some CI failures now, if you have a chance to look. @TomAugspurger I'm not sure how to fix it. See https://github.com/pandas-dev/pandas/pull/29334#discussion_r364716860 Thanks for the update. I'm not sure either :/ @howsiwei is this still active? Can you fix merge conflicts and try to get CI green?

Also want to move whatsnew to 1.1.0 at this point @WillAyd I'm not sure how to proceed due to https://github.com/pandas-dev/pandas/pull/29334#discussion_r364716860. I can get the CI green with my previous approach (https://github.com/pandas-dev/pandas/blob/8ccd2b879b1a10beb7760f1f1b43fac36b41ae19/pandas/core/indexing.py#L1731) of having different checks depending on whether the DataFrame is set by `.loc[]` or `[]` but that was advised against by @jreback. > @WillAyd I'm not sure how to proceed due to [#29334 (comment)](https://github.com/pandas-dev/pandas/pull/29334#discussion_r364716860). I can get the CI green with my previous approach (
> 
> https://github.com/pandas-dev/pandas/blob/8ccd2b879b1a10beb7760f1f1b43fac36b41ae19/pandas/core/indexing.py#L1731
> 
> ) of having different checks depending on whether the DataFrame is set by `.loc[]` or `[]` but that was advised against by @jreback.

you can pass axis @howsiwei can you merge master to resolve conflicts @howsiwei can you fix up merge conflict? Otherwise should be good to merge here thanks @howsiwei lgtm.

@jbrockmendel if any comments. @howsiwei this looks good. can you merge master and ping on green (just to make sure) @jreback merged master. It's green now. thanks @howsiwei very nice
thanks for sticking with it!