Document http interface's need for urlencode(dict)

This is to accomondate existing customers who accidentally depend on such behavior.
It also documents the future direction of using an already-encoded string,
so that such expectation is clear in this first release of this http interface.
In the future, when we switch existing dict behavior to string behavior,
it won't be considered as a breaking change.

Author: rayluo

oauth2cli/http.py

@@ -11,7 +11,14 @@ class HttpClient(object):
     def post(self, url, params=None, data=None, headers=None, **kwargs):
         """HTTP post.
 
-        params, data and headers MUST accept a dictionary.
+        :param dict params: A dict to be url-encoded and sent as query-string.
+        :param dict headers: A dict representing headers to be sent via request.
+        :param data:
+            Implementation needs to support 2 types.
+
+            * A dict, which will need to be urlencode() before being sent.
+            * (Recommended) A string, which will be sent in request as-is.
+
         It returns an :class:`~Response`-like object.
 
         Note: In its async counterpart, this method would be defined as async.
@@ -21,7 +28,9 @@ def post(self, url, params=None, data=None, headers=None, **kwargs):
     def get(self, url, params=None, headers=None, **kwargs):
         """HTTP get.
 
-        params, data and headers MUST accept a dictionary.
+        :param dict params: A dict to be url-encoded and sent as query-string.
+        :param dict headers: A dict representing headers to be sent via request.
+
         It returns an :class:`~Response`-like object.
 
         Note: In its async counterpart, this method would be defined as async.