[server] Move parsing of DXF parameters in QgsWmsParameters...

[pblottiere]

Description

...to be consistent with other WMS requests.

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include fixes #11111 in the commit message next to the description
• Commits which add new features are tagged with [FEATURE] in the commit message
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• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
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[pblottiere]

Considering that we do not have unit tests for DXF format (which is a shame :(), @rldhont @elpaso @mhugent, could you do a quick review on this PR please? It's mainly copy/paste.

[pblottiere]

I added some unit tests on the getDxf method of the renderer, so I'm more confident :).

I'll merge this PR in the coming days if I don't have any comments.