Change private methods to protected visibility

[phillmac]

Enables extension of the RequestDataTransformer class by allowing implementation of a custom transformField function

[codecov]

Codecov Report

Merging #29 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #29   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity      190    190           
=======================================
  Files             7      7           
  Lines           448    448           
=======================================
  Hits            448    448

Impacted Files	Coverage Δ	Complexity Δ
src/RequestDataTransformer.php	100% <100%> (ø)	25 <6> (ø)	⬇️

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