Fix self.headers got modified during request

[keatonLiu]

If request with json parameter set, function execute_request will not use the inferred content_type even if data is None.
becuase of the following two lines:

# set content type if it isn't set
if content_type is not None and "content-type" not in self.headers:
    self.headers["Content-Type"] = content_type

If the inferred content-type of the previous request is 'x-www-url-form-encoded', then this request will also be set to 'x-www-url-form-encoded' because self.headers got modified, which is not expected.

We should check the content-type after the headers are processed(merged), then we can use the CaseInsensitiveDict feature to check if "content-type" is set, if not, use the inferred one.