Make handler.characters do as vnu code expects.

Fixes https://github.com/validator/validator/issues/2

The vnu message-constructing code attempts to auto-capitalize the first word of certain types of messages it creates. That code needs/expects/assumes that the handler.characters method it calls can emit a substring of the character array it receives as input—instead of always just returning the entire array as-is. For the relevant code, see https://github.com/validator/validator/blob/master/src/nu/validator/messages/MessageEmitterAdapter.java#L1118 and the surrounding lines.