Return parameters enumerator from transform_keys/!

Previously calling `ActionController::Parameters#transform_keys/!`
without passing a block would return an enumerator for the underlying
hash, which was inconsistent with the behaviour when a block was passed:

    ActionController::Parameters.new(foo: "bar").transform_keys { |k| k }
    => <ActionController::Parameters {"foo"=>"bar"} permitted: false>
    ActionController::Parameters.new(foo: "bar").transform_keys.each { |k| k }
    => {"foo"=>"bar"}

An enumerator for the parameters is now returned instead, ensuring that
evaluating it produces another parameters object instead of a hash:

    ActionController::Parameters.new(foo: "bar").transform_keys.each { |k| k }
    => <ActionController::Parameters {"foo"=>"bar"} permitted: false>

1 files changed, 5 insertions, 0 deletions

diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index d9046f1431..32721ac233 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Calling `ActionController::Parameters#transform_keys/!` without a block now returns
+    an enumerator for the parameters instead of the underlying hash.
+
+    *Eugene Kenny*
+
 * Fix strong parameters blocks all attributes even when only some keys are invalid (non-numerical). It should only block invalid key's values instead.
 
     *Stan Lo*