Increase tolerance of addRangeAtPathChangeListener

This addresses a problem where the value at the end of a path passes
ephemerally through an invalid state for a range change listener, for
example listening for range changes on the "array" property, when the
property is temporarily a Map or null, until some other binding settles.
Merge pull request #1301 from kriskowal/tolerant-range-at-path

Increase tolerance of addRangeAtPathChangeListener