code: public Boolean getAsNullableBoolean(String key) {
  		Object value = getAsObject(key);
  		return BooleanConverter.toNullableBoolean(value);
  	}
docs: Converts map element into a boolean or returns null if conversion is not
  possible.
  
  @param key a key of element to get.
  @return boolean value of the element or null if conversion is not supported.
  
  @see BooleanConverter#toNullableBoolean(Object)