Add test for #327

MySQLdb returns unicode strings from a column with utf8_bin collation as bytestrings. This causes a `UnicodeDecodeError` when the string is passed to `unicode()`. The problem presents in tastypie when converting `field.CharField` values. This test covers that case by creating a bytestring with unicode characters and passing it to `CharField.convert()`.

This relates to issue #327 and pull request #768