code: public BsonValue get(final Object key, final BsonValue defaultValue) {
          BsonValue value = get(key);
          return value != null ? value : defaultValue;
      }
docs: If the document does not contain the given key, return the given default value.  Otherwise, gets the value of the key.
  
  @param key the key
  @param defaultValue the default value
  @return the value of the key as a BsonValue