Fix parameter name decoding when using POST + urlencoded

[peppsac]

Description

This is a followup of dfe48d1 which solved the issue for parameter values.

Fix https://issues.qgis.org/issues/17694

Checklist

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[pblottiere]

Nice, thank you @peppsac!

Can you just update your commit message with something like Fixes #17694 ...? This way the corresponding issue will be automatically closed once this PR merged.

[pblottiere]

Can you just update your commit message with something like Fixes #17694 ...? This way the corresponding issue will be automatically closed once this PR merged.

My apologies, it's already done 👍

[peppsac]

There's a similar problem to fix: + in parameters aren't converted to space (eg: LAYERS=my+layer should become LAYERS=my layer).
Do you me to push a 2nd commit to fix this in this PR? Or should I open a different PR?

[pblottiere]

Do you me to push a 2nd commit to fix this in this PR? Or should I open a different PR?

If it's linked to POST method and url encoding too, then I'd say that doing this fix in this PR is OK.

Otherwise, another PR would be welcomed :)

[peppsac]

Ok since it's exactly at the same place I amended the commit to fix the + to space conversion as well.