Updated TextMapPivotVectorizerTest to use OpEstimatorSpec

[tovbinm]

Related issues

TextMapPivotVectorizerTest is not using OpEstimatorSpec

Describe the proposed solution

1. Updated TextMapPivotVectorizerTest to use OpEstimatorSpec
2. Remove VectorizerDefaults mixin from TextMapPivotVectorizer and TextMapPivotVectorizerModel

[wsuchy]

LGTM

[codecov]

Codecov Report

Merging #290 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #290      +/-   ##
==========================================
+ Coverage    86.4%   86.42%   +0.01%     
==========================================
  Files         319      319              
  Lines       10453    10452       -1     
  Branches      351      346       -5     
==========================================
+ Hits         9032     9033       +1     
+ Misses       1421     1419       -2

Impacted Files	Coverage Δ
...scala/com/salesforce/op/test/OpEstimatorSpec.scala	93.33% <ø> (-0.22%)	⬇️
...p/stages/impl/feature/TextMapPivotVectorizer.scala	100% <ø> (ø)	⬆️
...es/src/main/scala/com/salesforce/op/OpParams.scala	89.79% <0%> (+4.08%)	⬆️

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