Fixes for discriminator

[justin-tay]

Closes #609, closes #588

This fixes issues with the discriminator and also adds support for processing in oneOf.

[codecov-commenter]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (2879ca3) 78.59% compared to head (391125e) 78.67%.

Files	Patch %	Lines
...a/com/networknt/schema/DiscriminatorValidator.java	76.19%	2 Missing and 3 partials ⚠️
...main/java/com/networknt/schema/OneOfValidator.java	86.66%	0 Missing and 4 partials ⚠️
...main/java/com/networknt/schema/JsonMetaSchema.java	80.00%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #971      +/-   ##
============================================
+ Coverage     78.59%   78.67%   +0.07%     
- Complexity     1810     1835      +25     
============================================
  Files           161      162       +1     
  Lines          5887     5941      +54     
  Branches       1119     1138      +19     
============================================
+ Hits           4627     4674      +47     
- Misses          833      835       +2     
- Partials        427      432       +5     

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