set default parameter to nil to speed up attribute_changed?

Benchmark results:

Warming up --------------------------------------
            old code    32.176k i/100ms
            new code    34.837k i/100ms
Calculating -------------------------------------
            old code      1.595M (± 3.5%) i/s -      7.947M
            new code      1.942M (± 3.9%) i/s -      9.685M

1 files changed, 5 insertions, 3 deletions

diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
index 6e2e5afd1b..fad4de9df5 100644
--- a/activemodel/lib/active_model/dirty.rb
+++ b/activemodel/lib/active_model/dirty.rb
@@ -174,10 +174,12 @@ module ActiveModel
     end
 
     # Handles <tt>*_changed?</tt> for +method_missing+.
-    def attribute_changed?(attr, options = {}) #:nodoc:
+    def attribute_changed?(attr, options = nil) #:nodoc:
       result = changes_include?(attr)
-      result &&= options[:to] == __send__(attr) if options.key?(:to)
-      result &&= options[:from] == changed_attributes[attr] if options.key?(:from)
+      if options
+        result &&= options[:to] == __send__(attr) if options.key?(:to)
+        result &&= options[:from] == changed_attributes[attr] if options.key?(:from)
+      end
       result
     end