fix invalid Not keyword resolution

[fmigneault]

fix invalid Not keyword resolution for LiteralInputType which resulted into too verbose process description inputs

The missing deserialization of the resulting mapping following OpenAPI not keyword validation resulted into all fields under a literal input type to bubble up in the process description. For example, fields like data_format, mode, mimetype, workdir, etc. that come from the pywps/owslib/cwl object json conversion would be returned.

[codecov]

Codecov Report

Merging #258 (a13dd93) into master (691c8b8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #258   +/-   ##
=======================================
  Coverage   71.80%   71.80%           
=======================================
  Files          61       61           
  Lines        9323     9323           
  Branches     1400     1400           
=======================================
  Hits         6694     6694           
  Misses       2237     2237           
  Partials      392      392           

Impacted Files	Coverage Δ
weaver/wps_restapi/colander_extras.py	77.41% <100.00%> (ø)
weaver/wps_restapi/swagger_definitions.py	99.88% <100.00%> (ø)

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