[java][jersey] Fix ALLOW_COERCION_OF_SCALARS

Created by: snago

Some minor cleanups and a bug fix for the Java Jersey2 & 3 client generators.

Fix a bug introduced in 8551b0af where ALLOW_COERCION_OF_SCALARS wasn't set properly on the ObjectMapper.

@wing328

cc @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)

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