Add support for recursive models

[peterjwest]

This is a port of noirbizarre/flask-restplus#656
It should resolve #110

Thanks to @buggyspace, @Drarok and @edwardfung123 for identifying and fixing this.

I've followed the CONTRIBUTING.rst file, but couldn't locate the changelog to add this change, is this out of date?

[codecov]

Codecov Report

Merging #174 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #174   +/-   ##
=======================================
  Coverage   96.92%   96.92%           
=======================================
  Files          19       19           
  Lines        2695     2697    +2     
=======================================
+ Hits         2612     2614    +2     
  Misses         83       83           

Impacted Files	Coverage Δ
flask_restx/swagger.py	96.74% <100.00%> (+0.01%)	⬆️

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[peterjwest]

@ziirish is there any chance you could review this?

[ziirish]

LGTM

[ziirish]

Thanks for your contribution!

We don't have a Changelog anymore.
We try to summarize the changes while making a new release based on the commit history.

[peterjwest]

Does this need to be rebased before it can be merged?

[j5awry]

@peterjwest yes, if you could give this a rebase. I'm giving my +1 as well, so we'll be able to merge after rebase.

[peterjwest]

@j5awry done