Only pass unicode multipart fieldnames to urllib3.

``filepost.encode_multipart_formdata`` says all fieldnames should be unicode and currently that is not the case - https://github.com/shazow/urllib3/blob/master/urllib3/filepost.py#L55 - since requests encodes to bytes here - https://github.com/kennethreitz/requests/blob/master/requests/models.py#L108.

Discovered after plenty of fun in https://github.com/requests/requests-oauthlib/pull/43. This should fix https://github.com/kennethreitz/requests/issues/1371.

Tested in 2.6, 2.7 and 3.3.

cc @Lukasa, @michaelhelmick, @sigmavirus24