Revert "Hash#symbolize_keys(!) optimizations"

Was slower in common case.

[#3891 state:open]

This reverts commit 2060977b767061a42eb8db2d5c3a30d205a94123.

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

@@ -22,7 +22,7 @@ def symbolize_keys
   # to +to_sym+.
   def symbolize_keys!
     keys.each do |key|
-      self[(key.to_sym rescue key)] = delete(key) if key.respond_to?(:to_sym) && !key.is_a?(Fixnum)
+      self[(key.to_sym rescue key) || key] = delete(key)
     end
     self
   end