add FRAGMENT_TYPE when customQuery and customMutation resolve interfaces

[Carnewal]

Hi there

Currently we have our interfaces accompanied with a __resolveType resolver that checks for the FRAGMENT_TYPE field. Currently, when using a custom mutation or query with a cypher, the FRAGMENT_TYPE is not added to the result, whereas when using fragments it is.

This PR solves this inconsistency by adding the first label as the FRAGMENT_TYPE if the schema type is an interface, similar to how fragments are handled.

This is a followup to #270

[codecov-io]

Codecov Report

Merging #296 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #296      +/-   ##
==========================================
+ Coverage   95.04%   95.05%   +<.01%     
==========================================
  Files          10       10              
  Lines        2341     2345       +4     
==========================================
+ Hits         2225     2229       +4     
  Misses        116      116

Impacted Files	Coverage Δ
src/utils.js	93.22% <100%> (+0.01%)	⬆️
src/translate.js	98.64% <100%> (ø)	⬆️

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[johnymontana]

Looks good. Thanks, @Carnewal!