Fix MappedSuperclass for JPA entity attributes

[igdianov]

This PR fixes class cast exception when mapping of @MappedSuperclass attribute from base class, i.e.

@ManyToOne(fetch= FetchType.LAZY)
SuperAuthor author;

[codecov]

Codecov Report

Merging #120 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #120      +/-   ##
============================================
+ Coverage     66.35%   66.37%   +0.01%     
- Complexity      379      381       +2     
============================================
  Files            34       34              
  Lines          2030     2031       +1     
  Branches        302      302              
============================================
+ Hits           1347     1348       +1     
  Misses          552      552              
  Partials        131      131

Impacted Files	Coverage Δ	Complexity Δ
...jpa/query/schema/impl/GraphQLJpaSchemaBuilder.java	87.29% <100%> (+0.02%)	115 <2> (+2)	⬆️

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