http_request method of Gitlab class should accept None as the default value for param post_data

[beyondliu]

http_request method of Gitlab class (in the file __init__.py) should accept None as the default value for param post_data.

For some caching servers, they reject HTTP GET request with body. If post_data's default value is {}, the request body of a GET request will get content b'{}' with Content-length header, which some caching server will just respond with an error. To maximize the support, I feel it suitable to change post_data's default value to None. Also it is not best practice in Python to use a mutable default value as an argument.

I didn't have time to read all the code. So let me know if this change is appropriate.