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.
- Add a version property to olx.format.WFSWriteTransactionOptions to set
the WFS protocol version to use
- Use the specified version to use the correct schema and version in the
GML request
- Use the version to select the proper GML serializer
