Added ability to customize/override 'consumes'

[courtarro]

I have needed the ability to override the Swagger spec to support "consumes" values other than application/json. I added this ability and a test to confirm that it works. It can be used like @app.consumes() or @app.doc(consumes=[]) just like how "produces" works.

[codecov[bot]]

Codecov Report

Merging #155 (76f66c8) into master (88497ce) will increase coverage by 0.06%. The diff coverage is 100.00%.

:exclamation: Current head 76f66c8 differs from pull request most recent head 35502c1. Consider uploading reports for the commit 35502c1 to get more accurate results

@@            Coverage Diff             @@
##           master     #155      +/-   ##
==========================================
+ Coverage   96.86%   96.92%   +0.06%     
==========================================
  Files          20       19       -1     
  Lines        2741     2700      -41     
==========================================
- Hits         2655     2617      -38     
+ Misses         86       83       -3     

Impacted Files	Coverage Δ
flask_restx/namespace.py	94.61% <100.00%> (+0.06%)	:arrow_up:
flask_restx/swagger.py	96.74% <100.00%> (ø)
flask_restx/resource.py	97.95% <0.00%> (-0.12%)	:arrow_down:
flask_restx/model.py	96.94% <0.00%> (-0.10%)	:arrow_down:
flask_restx/__init__.py
flask_restx/reqparse.py	98.72% <0.00%> (+<0.01%)	:arrow_up:
flask_restx/fields.py	96.59% <0.00%> (+0.13%)	:arrow_up:
flask_restx/api.py	97.13% <0.00%> (+0.36%)	:arrow_up:

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[zsuhor]

Would be very nice to get this merged and released.