Fix application/json support for PhoneID.

[ramsay]

The gson library doesn't handle nested HashMap objects without intervention. This changes the createRequestBody such that it will serialize the addons parameter separately than add to it all other simple key/value pairs from the params HashMap.

[codecov-io]

Codecov Report

Merging #62 into master will increase coverage by 5.96%.
The diff coverage is 86.95%.

@@             Coverage Diff              @@
##             master      #62      +/-   ##
============================================
+ Coverage     61.93%   67.89%   +5.96%     
- Complexity       31       39       +8     
============================================
  Files             7        7              
  Lines           176      190      +14     
  Branches         17       19       +2     
============================================
+ Hits            109      129      +20     
+ Misses           61       53       -8     
- Partials          6        8       +2

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/com/telesign/RestClient.java	89.83% <100%> (ø)	30 <4> (+1)	⬆️
src/main/java/com/telesign/PhoneIdClient.java	80% <83.33%> (+80%)	7 <4> (+7)	⬆️

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