Serialization feature for writing BigDecimal in plain form

In Java, BigDecimal.toString() writes out numbers in engineering form in certain cases. When this happens during serialization, the output may not be compatible with all JSON parsers.

* new BigDecimal("0.0").toString() gives "0.0"
* new BigDecimal("0.0000000").toString() gives "0E-8"

When enabled, the SerializationFeature I added prevents BigDecimal to use the scientific form.