Match the entire string when testing

This allows for validating that a pattern will fail to match an entire
string, even if it would match partially.

Fixes #114

Author: elyscape

spec/spec_helper.rb

@@ -19,7 +19,7 @@ def grok_match(label, message)
   end
 
   def build_grok(label)
-    grok = LogStash::Filters::Grok.new("match" => ["message", "%{#{label}}"])
+    grok = LogStash::Filters::Grok.new("match" => ["message", "\A%{#{label}}\z"])
     grok.register
     grok
   end