[java][jersey2-client] Disable coercion of scalars

[bkabrda]

I have a schema that has a oneOf with two choices, one is an alias for string and the other for int:

components:
  schemas:
    Something:
      type: object
      oneOf:
        - $ref: '#/components/schemas/One'
        - $ref: '#/components/schemas/Two'
    One:
      type: string
    Two:
      type: integer

The problem is that when the API returns a string that has integer in it, e.g. "1234", the Jackson library (which has coercion of scalars turned on by default) fails to deserialize the Something object, as "1234" deserializes successfully as both Integer and String. This PR disables scalar coercion to prevent this, thus making the above example work. (As an additional note, I think we want to have this feature disabled since it is something that other clients don't do)

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@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) @bkabrda (2020/01)

[bkabrda]

Hmm, actually there needs to be more work done in order for this setting to be respected inside the custom deserializers for oneOf. I'll add a another commit doing that.

[wing328]

Local test results look good.