CDRIVER-5759 fix relaxed encoding of date after year 9999

[kevinAlbs]

Fix encoding of date after year 9999 to agree with the Extended JSON specification.

Verified with this patch build.

Background & Motivation

The extended JSON spec notes:

BSON 1.1 Type or Convention	Canonical Extended JSON Format	Relaxed Extended JSON Format
...
Datetime [year before 1970 or after 9999]	{"$date": {"$numberLong": <64-bit signed integer giving millisecs relative to the epoch, as a string>}}	<Same as Canonical Extended JSON>

Includes a regression test from mongodb/specifications#1676.