REST API: Pass "null" as the post date property to reset post to initial "floating" date value.

Props TimothyBlynJacobs, adamsilverstein, jnylen0, mnelson4.
Fixes #44975.


Built from https://develop.svn.wordpress.org/trunk@46249


git-svn-id: http://core.svn.wordpress.org/trunk@46061 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api/class-wp-rest-request.php

@@ -397,6 +397,30 @@ class WP_REST_Request implements ArrayAccess {
 		return null;
 	}
 
+	/**
+	 * Checks if a parameter exists in the request.
+	 *
+	 * This allows distinguishing between an omitted parameter,
+	 * and a parameter specifically set to null.
+	 *
+	 * @since 5.3.0
+	 *
+	 * @param string $key Parameter name.
+	 *
+	 * @return bool True if a param exists for the given key.
+	 */
+	public function has_param( $key ) {
+		$order = $this->get_parameter_order();
+
+		foreach ( $order as $type ) {
+			if ( array_key_exists( $key, $this->params[ $type ] ) ) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+
 	/**
 	 * Sets a parameter on the request.
 	 *