Some WFS servers like tinyOWS require the feature prefix to be in the
feature type for the feature to be found (eg `prefix:layer`).
The problem was, the prefix was always added to the feature type which
gave us a type name like `prefix:prefix:feature` or
`feature:prefix:feature`. The requests were then rejected by the WFS
server.
We now check if the feature type starts with the prefix. If it does, we
don't append it again. If it doesn't we do.
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