$watchCollection checks if oldValue !== newValue which does not work for NaN. This was causing
infinite digest errors, since comparing NaN to NaN in $watchCollection would always return false,
indicating that a change was occuring on each loop.
This fix adds a simple check to see if the current value and previous value are both NaN, and
if so, does not count it as a change.
Closes#4605
fb6062fb9d