Fix $ref With Path Items

[zach-hamm]

I opened up an issue for a bug with $ref on path items. This PR applies the changes mentioned there to hopefully resolve the issue.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e9b6edf) 100.00% compared to head (933ffc8) 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #1323     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           11        26     +15     
  Lines         1020      3334   +2314     
  Branches       201       792    +591     
===========================================
+ Hits          1020      3334   +2314     

Flag	Coverage Δ
unittests	99.94% <99.92%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datamodel_code_generator/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/__main__.py	100.00% <100.00%> (ø)
datamodel_code_generator/format.py	100.00% <100.00%> (ø)
datamodel_code_generator/http.py	100.00% <100.00%> (ø)
datamodel_code_generator/imports.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/base.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/dataclass.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/enum.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/improts.py	100.00% <100.00%> (ø)
... and 16 more

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[koxudaxi]

@zach-hamm
I have released the new version as 0.20.0
Thank you very much!!