JsonNodeEquivalence: use Double for numeric values' hash code

Two BigDecimals with equivalent mathematical values will return the same double,
and will therefore have the same hashcode. This is what we want. And this is
faster than going through Big{Integer,Decimal}.

Of course, this means we trust Jackson to do the Good Thing(tm) with JsonNode's
.doubleValue().

Link: http://stackoverflow.com/questions/14311644/normalizing-bigdecimals-hash-code-howto

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>