Use String#sub instead of String#sub!

Because String#sub! is a destructive method.
Built-in in_tail plugin will configure the parse section twice.
The in_tail plugin will use raw parse section at the 1st time and it
will use parse section with expanded grok pattern without types.

We can avoid this behavior when we stop using a destructive method to
expand grok patterns.

Signed-off-by: Kenji Okimoto <okimoto@clear-code.com>

Author: okkez

lib/fluent/plugin/grok.rb

@@ -131,7 +131,7 @@ def expand_pattern(pattern)
         else
           replacement_pattern = "(?:#{curr_pattern})"
         end
-        pattern.sub!(m[0]) do |s|
+        pattern = pattern.sub(m[0]) do |s|
           replacement_pattern
         end
       end