validate parameter defaults in swagger 2.0 specs

[dtkav]

In Swagger 2.0 specs there can be defaults on the parameter object directly (i.e. not in the parameter schema).
Fixes #36

[codecov]

Codecov Report

Merging #37 into master will increase coverage by 0.29%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #37      +/-   ##
=========================================
+ Coverage    98.2%   98.5%   +0.29%     
=========================================
  Files          13      13              
  Lines         390     400      +10     
=========================================
+ Hits          383     394      +11     
+ Misses          7       6       -1

Impacted Files	Coverage Δ
openapi_spec_validator/validators.py	98.96% <100%> (+0.6%)	⬆️

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[p1c2u]

LGTM