xml_mini.rb:_dasherize() replacement 20 to 25% faster

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

activesupport/lib/active_support/xml_mini.rb

@@ -139,10 +139,9 @@ def rename_key(key, options = {})
     protected
 
     def _dasherize(key)
-      left  = key.strip.rpartition(/^_*/)
-      right = left.pop.partition(/_*$/)
-      right.first.tr!('_ ', '--')
-      left.concat(right).join
+      # $2 must be a non-greedy regex for this to work
+      left, middle, right = /\A(_*)(.*?)(_*)\Z/.match(key.strip)[1,3]
+      "#{left}#{middle.tr('_', '-')}#{right}"
     end
 
 	  # TODO: Add support for other encodings