ignore trailing whitespace in bson_json_reader_read

Fix for https://jira.mongodb.org/browse/CDRIVER-291

Ignores trailing whitespace in bson_json_reader_read (so json files made with editors don't choke).

Basically, it checks to see if a given call to read only processes whitespace bytes, in which case a parse error from yajl means not doc found.  Stops checking after a non-whitespace character is found for the given read.