Remove usage of internal OkHttp API.

[jaynewstrom-okta]

Closes okta/okta-auth-java#132

[codecov-commenter]

Codecov Report

Merging #72 (db06189) into master (67da074) will decrease coverage by 0.05%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master      #72      +/-   ##
============================================
- Coverage     63.73%   63.67%   -0.06%     
  Complexity      768      768              
============================================
  Files            39       39              
  Lines          2741     2745       +4     
  Branches        510      510              
============================================
+ Hits           1747     1748       +1     
- Misses          810      813       +3     
  Partials        184      184              

Impacted Files	Coverage Δ
...kta/commons/http/okhttp/OkHttpRequestExecutor.java	88.29% <50.00%> (-2.82%)	⬇️

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