Options: Add 'default' to register_setting

Add a `default` argument to `register_setting` that will be used an the default option value viet `get_option()` in the event of no other option being specified. This means (if chosen) developers can define their default once via `register_option` and not have to duplicate the value every time they make a call to `get_option()`.

Props rmccue, jorbin, jtsternberg.
Fixes #38176.


git-svn-id: https://develop.svn.wordpress.org/trunk@38910 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/option/registration.php

@@ -39,6 +39,28 @@ class Tests_Option_Registration extends WP_UnitTestCase {
 		return 'S-M-R-T';
 	}
 
+	/**
+	 * @ticket 38176
+	 */
+	public function test_register_with_default() {
+		register_setting( 'test_group', 'test_default', array(
+			'default' => 'Fuck Cancer'
+		));
+
+		$this->assertEquals( 'Fuck Cancer', get_option( 'test_default' ) );
+	}
+
+	/**
+	 * @ticket 38176
+	 */
+	public function test_register_with_default_override() {
+		register_setting( 'test_group', 'test_default', array(
+			'default' => 'Fuck Cancer'
+		));
+
+		$this->assertEquals( 'Fuck Leukemia', get_option( 'test_default', 'Fuck Leukemia' ) );
+	}
+
 	/**
 	 * @expectedDeprecated register_setting
 	 */