fix: update content-length header for next page

The content-length header only gets updated at request object
initialization, so make sure we update it here after we modified the
body.

Fixes googleapis#1403

googleapiclient/discovery.py

@@ -1283,6 +1283,9 @@ def methodNext(self, previous_request, previous_response):
             body = model.deserialize(request.body)
             body[pageTokenName] = nextPageToken
             request.body = model.serialize(body)
+            request.body_size = len(request.body)
+            if "content-length" in request.headers:
+              del request.headers['content-length']
             logger.debug("Next page request body: %s %s" % (methodName, body))
 
         return request