Entering negatives into an <input type="number">

When entering a negative number into a value field. The absolute value
would instead be set. This is because values are entered char-by-char
and '-' by itself wasn't considered a valid value.

This modifies PoltergeistAgent.Node.set to set the entire value at once
for an input type=number. Number fields aren't necessarily set via
keypresses anyways (they have those little buttons).

Includes a failing test case.

This change may be unnecessary if native keyboard events are used
again (landed in c7ba561, reverted in 70304cc).