[Python] Allow Unicode Characters in Request Body.

In order to send a request with a string body containing unicode characters (like '∆', for example), the body needs to be explicitly encoded using body_str.encode('utf-8') in order avoid the UnicodeEncodeError in https://github.com/python/cpython/blob/master/Lib/http/client.py#L1313

1312        if isinstance(body, str):
1313            # RFC 2616 Section 3.7.1 says that text default has a
1314            # default charset of iso-8859-1.
1315            body = _encode(body, 'body')

which is gonna try to explicitly encode it as latin-1 and fail.

I propose to fix it by updating this condition here
https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/python/rest.mustache#L182
which should allow all string types, as well as bytes type

182                elif isinstance(body, (six.string_types, six.binary_type))

instead of checking just for isinstance of str

Start at modules/swagger-codegen/src/main/resources/python/rest.mustache around line 182 and review how generated Python clients classify request bodies. Verify the generated client can send Unicode string and bytes bodies without the reported encoding failure; no test file is named in the issue.