Inline the symbolize_keys/stringify_keys methods

user     system      total        real
symbolize_keys  5.980000   0.070000   6.050000 (  6.048187)
new_symbolize_keys  4.310000   0.050000   4.360000 (  4.364745)

activesupport/lib/active_support/core_ext/hash/keys.rb

 class Hash
   # Return a new hash with all keys converted to strings.
   def stringify_keys
-    dup.stringify_keys!
+    result = {}
+    keys.each do |key|
+      result[key.to_s] = self[key]
+    end
+    result
   end
 
   # Destructively convert all keys to strings.
@@ -15,7 +19,11 @@ def stringify_keys!
   # Return a new hash with all keys converted to symbols, as long as
   # they respond to +to_sym+.
   def symbolize_keys
-    dup.symbolize_keys!
+    result = {}
+    keys.each do |key|
+      result[(key.to_sym rescue key)] = self[key]
+    end
+    result
   end
 
   # Destructively convert all keys to symbols, as long as they respond