Fix MemoryStore#write with unless_exist and namespace

Updates `MemoryStore#write_entry` to pass a `nil` `namespace` to
`exist?`, which expects a _name_ rather than a an already "normalized"
_key_. This fixes a bug where `unless_exist` would overwrite any
existing entry if a `namespace` was used.

activesupport/lib/active_support/cache/memory_store.rb

@@ -210,7 +210,7 @@ module ActiveSupport
         def write_entry(key, entry, **options)
           payload = serialize_entry(entry, **options)
           synchronize do
-            return false if options[:unless_exist] && exist?(key)
+            return false if options[:unless_exist] && exist?(key, namespace: nil)
 
             old_payload = @data[key]
             if old_payload

activesupport/test/cache/stores/memory_store_test.rb

@@ -61,6 +61,28 @@ class MemoryStoreTest < ActiveSupport::TestCase
     assert_same value, @cache.read("key")
   end
 
+  def test_write_with_unless_exist
+    assert_equal true, @cache.write(1, "aaaaaaaaaa")
+    assert_equal false, @cache.write(1, "aaaaaaaaaa", unless_exist: true)
+    @cache.write(1, nil)
+    assert_equal false, @cache.write(1, "aaaaaaaaaa", unless_exist: true)
+  end
+
+  def test_namespaced_write_with_unless_exist
+    namespaced_cache = lookup_store(expires_in: 60, namespace: "foo")
+
+    assert_equal true, namespaced_cache.write(1, "aaaaaaaaaa")
+    assert_equal false, namespaced_cache.write(1, "aaaaaaaaaa", unless_exist: true)
+    namespaced_cache.write(1, nil)
+    assert_equal false, namespaced_cache.write(1, "aaaaaaaaaa", unless_exist: true)
+  end
+
+  def test_write_expired_value_with_unless_exist
+    assert_equal true, @cache.write(1, "aaaa", expires_in: 1.second)
+    travel 2.seconds
+    assert_equal true, @cache.write(1, "bbbb", expires_in: 1.second, unless_exist: true)
+  end
+
   private
     def compression_always_disabled_by_default?
       true
@@ -187,17 +209,4 @@ class MemoryStorePruningTest < ActiveSupport::TestCase
     assert_not_equal item.object_id, read_item.object_id
     assert_not_equal read_item.object_id, @cache.read(key).object_id
   end
-
-  def test_write_with_unless_exist
-    assert_equal true, @cache.write(1, "aaaaaaaaaa")
-    assert_equal false, @cache.write(1, "aaaaaaaaaa", unless_exist: true)
-    @cache.write(1, nil)
-    assert_equal false, @cache.write(1, "aaaaaaaaaa", unless_exist: true)
-  end
-
-  def test_write_expired_value_with_unless_exist
-    assert_equal true, @cache.write(1, "aaaa", expires_in: 1.second)
-    travel 2.seconds
-    assert_equal true, @cache.write(1, "bbbb", expires_in: 1.second, unless_exist: true)
-  end
 end