Small fix for logstash/event.rb's sprintf method.

I found this with the riemann output - if you set it up with:
riemann_event => [ 'metric', 0 ]

Then the value gets sprintf'd, which utterly fails in this unquoted case, as 0
is a Fixnum, which doesn't have an index method.

This, we explicitly call .to_s in the sprintf method, to ensure that the
format is a string before call .index on it
Merge pull request #559 from bobtfish/event_sprintf_fixnums

Small fix for logstash/event.rb's sprintf method.