Deep duplicate in `deep_merge` so no references remain to the original hash in the result

When deep merging a Hash, first a duplicate is created. The duplicate only
does a shallow copy, so deeper level structures are not duplicated but
remain references.

This can lead to unexpected modifications of the original hash when a deeply
nested hash is merged with another hash, and the newly returned hash gets modified.
```
  x = { a: { b: "foo" } }
  y = { d: { e: "bar" } }

  z = x.deep_merge(y)
  # z => { a: { b: "foo" }, d: { e: "bar" } }

  z[:a][:b] = "baz"
  # z => { a: { b: "baz" }, d: { e: "bar" } }
  # x => { a: { b: "baz" } }
```

activesupport/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   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)
-    dup.deep_merge!(other_hash, &block)
+    deep_dup.deep_merge!(other_hash, &block)
   end
 
   # Same as +deep_merge+, but modifies +self+.

activesupport/test/core_ext/hash_ext_test.rb

@@ -300,6 +300,16 @@ 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 }