Unicode strings in multipart post requests

When attempting to post a file along with some other data including a unicode string (demonstrated in the added test case) I was running into the following error:

UnicodeEncodeError: 'ascii' codec can't encode character u'\xeb' in position 0: ordinal not in range(128)

I traced the issue back to `_encode_files` where `compat.builtin_str`, in my case `str`, was being applied to the unicode string causing the above error. I changed it to use `compat.str` which seems to have resolved the issue.

``` bash
> python --version
Python 2.7.2
```