feat(ngMaxlength): add support for disabling max length limit

Previously, setting the maxlength to a negative number, would make all
input values invalid (since their length should be less than maxlength,
which is impossible).
This commit changes the behaviour of maxlength/ngMaxlength, effectively
disabling the maxlength validation (always returning true) when maxlength
is set to a negative number. This is more inline to how the HTML5
`maxlength` attribute works (both in browsers and according to the spec:
http://dev.w3.org/html5/spec-preview/attributes-common-to-form-controls.html#attr-fe-maxlength).

Related to #9874
Closes #9995
fix(ngMaxlength): ignore maxlength when not set to a non-negative integer

This makes the behaviour of maxlength/ngMaxlength more inline with the
spec.

Closes #9874
docs(ngMaxlength): document what happens on negative/non-numeric values

Closes #9998