Fixing unregistered nested ordered model bug

[andre-c-andersen]

When setting ordered=True in a Namespace, nested models are not registered with Swagger giving a "Could not resolve reference" in Swagger documentation. This pull request fixes this.

Related bug reports and pull requests:

• https://stackoverflow.com/questions/56079764/flask-restplus-nested-fields-not-getting-registered-into-swaggerui
• Nested Fields not getting registrated into swagger noirbizarre/flask-restplus#643
• noirbizarre/flask-restplus@847664e
• Api(..., ordered=True) and Nested models → error in Swagger UI #128

[codecov]

Codecov Report

Merging #267 (61cdc3b) into master (b31bd79) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #267   +/-   ##
=======================================
  Coverage   96.86%   96.86%           
=======================================
  Files          19       19           
  Lines        2712     2712           
=======================================
  Hits         2627     2627           
  Misses         85       85           

Impacted Files	Coverage Δ
flask_restx/swagger.py	96.74% <100.00%> (ø)

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[ziirish]

Great, thanks!