bugfix: custom mutation cypher params not being passed

[a-type]

When writing a custom Cypher mutation using the @cypher directive, cypherParams would not be passed down into field selections with nested @cypher directives. It seems that the cypherParams parameter was omitted from buildCypherSelection (unlike customQuery, which does pass it properly).

Added a unit test to avoid regressions.

[codecov-io]

Codecov Report

Merging #246 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #246   +/-   ##
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  Coverage   94.47%   94.47%           
=======================================
  Files           4        4           
  Lines         326      326           
=======================================
  Hits          308      308           
  Misses         18       18

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[johnymontana]

Looks good!

[johnymontana]

Thanks @a-type!