XContentBuilder.writeValue() serializes java.math.BigDecimal as String

[nickivanov]

When adding documents to the index that are represented by Map<String, Object>, and the documents contain instances of java.math.BigDecimal or java.math.BigInteger, they are serialized as strings, and are subsequently mapped by Elasticsearch to string fields.
Although java.math.BigDecimal and java.math.BigInteger are supported by Jackson, XContentBuilder.writeValue() defaults to represent these data types as strings.

[clintongormley]

Lucene currently has no support for BigInteger/Decimal.

Closing in favour of #5683