refactor(HTTPError): Add a "to_" prefix to serialization methods

Perfix the serialization method names (.dict, .json, .xml) with
"to_" in order to more clearly express their utility.
Merge pull request #359 from kgriffs/rename_error_helpers

refactor(HTTPError): Add a "to_" prefix to serialization methods
test(req.uri): Test HTTPS URIs

Add tests to cover HTTPS URIs using default and non-default ports.