Fix HWIA#transform_keys! removing defaults

A [previous change][1] fixed `HWIA#transform_keys!` from clobbering keys
if they ended up transforming into another pre-transformed key. However,
that change also resulted in `HWIA#transform_keys!` clearing the
`default`/`default_proc`.

This commit fixes that issue and adds test coverage to both
`#transform_keys` and `#transform_keys!` to ensure that their behavior
with regard to `default`/`default_proc` is consistent with `Hash`.

```irb
irb(main):001> Hash.new(:a).transform_keys(&:to_s).default
=> nil
irb(main):002> Hash.new(:a).transform_keys!(&:to_s).default
=> :a
```

[1]: f49d7a0

Author: skipkayhil

activesupport/lib/active_support/hash_with_indifferent_access.rb

@@ -368,12 +368,12 @@ def transform_keys(hash = NOT_GIVEN, &block)
     def transform_keys!(hash = NOT_GIVEN, &block)
       if NOT_GIVEN.equal?(hash)
         if block_given?
-          replace(transform_keys(&block))
+          replace(transform_keys(&block).tap { |h| set_defaults(h) })
         else
           return to_enum(:transform_keys!)
         end
       else
-        replace(transform_keys(hash, &block))
+        replace(transform_keys(hash, &block).tap { |h| set_defaults(h) })
       end
 
       self

activesupport/test/hash_with_indifferent_access_test.rb

@@ -486,6 +486,18 @@ def test_indifferent_transform_keys
     assert_raise TypeError do
       hash.transform_keys(nil)
     end
+
+    hash_with_default = Hash.new(:a)
+    hash = ActiveSupport::HashWithIndifferentAccess.new(hash_with_default).transform_keys(&:to_s)
+    assert_nil hash.default
+    hash = ActiveSupport::HashWithIndifferentAccess.new(hash_with_default).transform_keys { |k| k.to_s }
+    assert_nil hash.default
+
+    hash_with_default_proc = Hash.new { |h, k| h[k] = :b }
+    hash = ActiveSupport::HashWithIndifferentAccess.new(hash_with_default_proc).transform_keys(&:to_s)
+    assert_nil hash.default_proc
+    hash = ActiveSupport::HashWithIndifferentAccess.new(hash_with_default_proc).transform_keys { |k| k.to_s }
+    assert_nil hash.default_proc
   end
 
   def test_indifferent_deep_transform_keys
@@ -540,6 +552,18 @@ def test_indifferent_transform_keys_bang
     assert_raise TypeError do
       hash.transform_keys(nil)
     end
+
+    hash_with_default = Hash.new(:a)
+    hash = ActiveSupport::HashWithIndifferentAccess.new(hash_with_default).transform_keys!(&:to_s)
+    assert_equal hash_with_default.default, hash.default
+    hash = ActiveSupport::HashWithIndifferentAccess.new(hash_with_default).transform_keys! { |k| k.to_s }
+    assert_equal hash_with_default.default, hash.default
+
+    hash_with_default_proc = Hash.new { |h, k| h[k] = :b }
+    hash = ActiveSupport::HashWithIndifferentAccess.new(hash_with_default_proc).transform_keys!(&:to_s)
+    assert_equal hash_with_default_proc.default_proc, hash.default_proc
+    hash = ActiveSupport::HashWithIndifferentAccess.new(hash_with_default_proc).transform_keys! { |k| k.to_s }
+    assert_equal hash_with_default_proc.default_proc, hash.default_proc
   end
 
   def test_indifferent_deep_transform_keys_bang