Add 2LO support for OAuth 1.0.

Reviewed in http://codereview.appspot.com/4517087/
diff --git a/apiclient/http.py b/apiclient/http.py
index 2481c23..5f4be00 100644
--- a/apiclient/http.py
+++ b/apiclient/http.py
@@ -218,6 +218,7 @@
   'echo_request_headers_as_json' means return the request headers in
      the response body
   'echo_request_body' means return the request body in the response body
+  'echo_request_uri' means return the request uri in the response body
   """
 
   def __init__(self, iterable):
@@ -240,6 +241,8 @@
       content = simplejson.dumps(headers)
     elif content == 'echo_request_body':
       content = body
+    elif content == 'echo_request_uri':
+      content = uri
     return httplib2.Response(resp), content