[Java][okhttp-gson] Making API response headers case insensitive

Created by: saigiridhar21

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Description of the PR

Closes:#3030 (closed)

Making API response headers case insensitive.

Okhttp's Headers' class has only one method called "toMultiMap" which returns headers as Map. So, it is better handle it separately in APIResponse and ApiException classes separately.

I will be raising separate pull requests for other libraries of Java.

@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04)