Reduce limit for turning GETs into POSTs to accomodate App Engine.

Reviewed in https://codereview.appspot.com/6852061/.
diff --git a/apiclient/http.py b/apiclient/http.py
index 569815a..d590d1b 100644
--- a/apiclient/http.py
+++ b/apiclient/http.py
@@ -51,7 +51,7 @@
 
 DEFAULT_CHUNK_SIZE = 512*1024
 
-MAX_URI_LENGTH = 4000
+MAX_URI_LENGTH = 2048
 
 
 class MediaUploadProgress(object):