Repair default and $ref issue

[hlobit]

Fixes #92 #98

[codecov]

Codecov Report

Merging #91 (0144b5b) into master (5521114) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #91   +/-   ##
=======================================
  Coverage   98.22%   98.23%           
=======================================
  Files          18       18           
  Lines         508      510    +2     
=======================================
+ Hits          499      501    +2     
  Misses          9        9           

Impacted Files	Coverage Δ
openapi_spec_validator/validators.py	99.06% <100.00%> (+<0.01%)	⬆️

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