Revert "Merge pull request #41931 from MarcelEeken/deep-merge-changing-original-hash"

This reverts commit 483d36ae09, reversing
changes made to b171b842da.

activesupport/CHANGELOG.md

@@ -13,10 +13,6 @@
 
     *Andrew White*
 
-*   Fix issue in `Hash#deep_merge` where it did not properly duplicate a nested `Hash`
-
-    *Marcel Eeken*
-
 *   Add `expires_at` argument to `ActiveSupport::Cache` `write` and `fetch` to set a cache entry TTL as an absolute time.
 
     ```ruby

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

@@ -16,7 +16,7 @@ class Hash
   #   h1.deep_merge(h2) { |key, this_val, other_val| this_val + other_val }
   #   # => { a: 100, b: 450, c: { c1: 300 } }
   def deep_merge(other_hash, &block)
-    deep_dup.deep_merge!(other_hash, &block)
+    dup.deep_merge!(other_hash, &block)
   end
 
   # Same as +deep_merge+, but modifies +self+.

activesupport/test/core_ext/hash_ext_test.rb

@@ -300,16 +300,6 @@ class HashExtTest < ActiveSupport::TestCase
     assert_equal expected, hash_1
   end
 
-  def test_deep_merge_with_nested_hash_returning_full_new_hash
-    hash_1 = { a: { b: "foo" } }
-    hash_2 = { d: "bar" }
-
-    new_hash = hash_1.deep_merge(hash_2)
-    new_hash[:a][:b] = "baz"
-
-    assert_equal("foo", hash_1[:a][:b])
-  end
-
   def test_reverse_merge
     defaults = { d: 0, a: "x", b: "y", c: 10 }.freeze
     options  = { a: 1, b: 2 }