fix: abstract classes should generate interface

[dariuszkuc]

Fixes: #220

[codecov]

Codecov Report

Merging #221 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #221   +/-   ##
=========================================
  Coverage     98.43%   98.43%           
- Complexity      178      179    +1     
=========================================
  Files            57       57           
  Lines           574      574           
  Branches         98       99    +1     
=========================================
  Hits            565      565           
  Misses            4        4           
  Partials          5        5

Impacted Files	Coverage Δ	Complexity Δ
...otlin/com/expedia/graphql/generator/TypeBuilder.kt	100% <100%> (ø)	20 <0> (+1)	⬆️
...dia/graphql/generator/filters/superclassFilters.kt	100% <100%> (ø)	0 <0> (ø)	⬇️

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