Don't double-unslash meta key when update_metadata() falls back on add_metadata().

Props jdgrimes.
Fixes #35795.

git-svn-id: https://develop.svn.wordpress.org/trunk@36509 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/meta/updateMetadata.php

@@ -0,0 +1,30 @@
+<?php
+
+/**
+ * @group meta
+ * @covers ::update_metadata
+ */
+class Tests_Meta_UpdateMetadata extends WP_UnitTestCase {
+	/**
+	 * @ticket 35795
+	 */
+	public function test_slashed_key_for_new_metadata() {
+		update_metadata( 'post', 123, wp_slash( 'foo\foo' ), 'bar' );
+
+		$found = get_metadata( 'post', 123, 'foo\foo', true );
+		$this->assertSame( 'bar', $found );
+	}
+
+	/**
+	 * @ticket 35795
+	 */
+	public function test_slashed_key_for_existing_metadata() {
+		global $wpdb;
+
+		add_metadata( 'post', 123, wp_slash( 'foo\foo' ), 'bar' );
+		update_metadata( 'post', 123, wp_slash( 'foo\foo' ), 'baz' );
+
+		$found = get_metadata( 'post', 123, 'foo\foo', true );
+		$this->assertSame( 'baz', $found );
+	}
+}