CDRIVER-402: bson_append_null should be used

bson_append_null should be used for marshaling a BSON_TYPE_NULL data
Merge pull request #85 from aksaharan/patch-1

CDRIVER-402: bson_append_null should be used
Merge pull request #84 from Machyne/master

doc: fix typo in bson_oid_is_valid comment header
bson: add check for non-shortest form during validation.
Fixed documentation typo
Merge pull request #86 from edaniels/master

Fixed documentation typo