Check _bsonType for Binary

This fixes a problem that I had which was very similar to http://stackoverflow.com/questions/11529458/mongodb-gridfs-illegal-chunk-format-exception?lq=1 except that I could not find any duplication.  

Unfortunately I couldn't get the tests to work (Error: ENOENT, no such file or directory '/home/mark/Projects/node-mongodb-native/data/data-27017/mongod.lock') even before I made the edit.