`multipleOf` is inconsistent, fails for negative numbers

[leongersen]

The specification doesn't say negative numbers should fail a multipleOf constraint, yet the current documentation fails -49.89 with a multipleOf value of 0.01.

Funnily enough, the implementation of this constraint has some edge cases where it lets negative numbers pass:

Calling the NumberConstraint::fmod function with the parameters:

• -49.89 and 0.01: returns -0.01, thus fails
• -49.88 and 0.01: returns -0, which passes due to the non-strict comparison in the NumberConstraint::check function.

Calling abs() on the first argument of NumberConstraint::fmod should both fix this inconsistency and bring multipleOf in line with the spec.