Support content-type overrides

[mattwwarren]

This may be related to #132 so feel free to fold it into that but we ran into a scenario where we had to pass a meaningless files= kwarg to HTTPClient.request in order to get the proper Content-Type header. Would you be open to only setting that header if one does not exist in the headers= kwarg?

Happy to make that change, just wanted to confirm my design beforehand.

[twm]

That sounds reasonable to me. I can see other use cases, e.g. using the json keyword argument with a JSON Patch content type.