allow reading maps from number -> value

allow reading maps from number -> value

Summary:
The serialization of PHP values often uses integer -> value
maps in JSON arrays. These are emitted by the standard stream <<
operator on dynamics but cannot be read. This diff fixes that.

Test Plan: - read in serialized value with array "bucketing":{"days_stale":{0:{2:null,1:14.01,0:"more_than_two_weeks_stale"}}}

Reviewed By: delong.j@fb.com

FB internal diff: D805218