fix: Content-Length measures the response body, not the object being read (#742)

Author: cjc25

gcs/object.py

@@ -535,7 +535,7 @@ def streamer():
             headers["Content-Type"] = "application/octet-stream"
 
         headers["x-goog-stored-content-length"] = self.metadata.size
-        headers["Content-Length"] = self.metadata.size
+        headers["Content-Length"] = len(response_payload)
 
         if self.metadata.content_encoding:
             headers["x-goog-stored-content-encoding"] = self.metadata.content_encoding