Add __isset() method to Relationship class

Without this, calling `isset()` on an overloaded property accessed via
the magic `__get()` method, like `public_id` returns `false`, even if
it is set.

Author: acrobertson

php/relate/class-relationship.php

@@ -117,6 +117,19 @@ public function __get( $key ) {
 		return $return;
 	}
 
+	/**
+	 * Check the existence of a relationship data value.
+	 *
+	 * @param string $key The key.
+	 *
+	 * @return bool
+	 */
+	public function __isset( $key ) {
+		$data_cache = $this->get_data();
+
+		return isset( $data_cache[ $key ] );
+	}
+
 	/**
 	 * Set the save on shutdown flag.
 	 */