Indicate Decimal128 is just the 128 version in extended JSON

While implementing HHVM-186 (Decimal128 support in the HHVM driver), bjori pointed out that I should name the class Decimal128 instead of Decimal.

When doing so, I noticed that the extended JSON representation, as well as the shell, do not include the specific type (128) either:

{ "field" : { "$numberDecimal" : "1234.5678" } }

Is there a reason why the "128" isn't included in either extendedJSON or shell wrapper object?