More relaxed decoding of graphite messages

Technically the graphite spec at the below URL only specifies space
chars between message fields.
http://graphite.wikidot.com/getting-your-data-into-graphite

However several senders use space and tab chars e.g. the
Sensu::Plugin::Metric::CLI::Graphite class uses tab characters between
fields and there is a large number of deployed hosts using that plugin,
so it may be appropriate to split on characters beyond a single space.

The accompanying tests have additional assertions to test for messages
that contain multiple tabs and spaces between fields.