Group metadata not updated when a yum proxy repository's remote URL is changed.

1. Set up a yum proxy with an incorrect URL, and put it in a yum group repository.   
2. Add a yum hosted repository to the group with metadata depth of 0. 
3. Upload an rpm to the hosted repository.  
4. Retrieve the repomd/repodata.xml file through the group.
5. Fix the proxy's remote URL, and download the repomd.xml file through the proxy.

Now try retrieving the the repomd/repodata.xml file through the group again. Observe that the metadata is not updated. Invalidating the group's cache does not fix this.

Expiring cache in either the proxy or group repository does not fix this.  Not sure if that is a separate bug, or if it is another symptom of the same bug.

The only workaround I have been able to find is to delete the group repository and recreate it.

I've attached an NXRM 3.18.0 work directory that is in this state.