Require URIs to be absolute to be valid. This fixes issue #122.
In RDF::Writer.dump/buffer/open: Set external_encoding based on :encoding option if it is not otherwise set. Also, if no :encoding option is given, but the IO/StringIO file has an external_encoding, use that as the default for output encoding. This relates to issue #111
In NTriples::Writer, If encoding is ASCII, do not use \b or \f output escapes.