Fix direct serialization of primitive types in DEFAULT_JSONP_MAPPER

[Xtansia]

Description

Fix direct serialization of primitive types in DEFAULT_JSONP_MAPPER as used by toJsonString().

Issues Resolved

Closes #1503

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[reta]

@Xtansia the DEFAULT_JSONP_MAPPER was not designed to be generic JSON mapper, it shouldn't be used with primitive types. Back to the issue: what is expectation of sending the index request (which is just a string) to OpenSearch? I would expect the document to a class instance of JSON type.

[Xtansia]

Have switched to a more sensible example. ExtendedBounds is generic and uses JsonpUtils.serialize, Which directly passes through to mapper.serialize if no explicit serializer is passed or type isn't JsonpSerializable. Which is the same mechanism that IndexRequest uses, just ExtendedBounds<Double> is an actively in-use type within the API types.

I don't think it's unreasonable to directly handle the primitives that map directly to JsonGenerator calls. The alternative is moving this logic into JsonpUtils.serialize.

[bdelbosc]

Hi, any feedback on this PR? SearchRequest#toJsonString is not usable without this fix.