fix(carddav): IAddressBook::getKey() should return a string

Signed-off-by: Richard Steinmetz <richard@steinmetz.cloud>

apps/dav/lib/CardDAV/AddressBookImpl.php

@@ -41,7 +41,7 @@ class AddressBookImpl implements IAddressBookEnabled {
 	 * @since 5.0.0
 	 */
 	public function getKey() {
-		return $this->addressBookInfo['id'];
+		return (string)$this->addressBookInfo['id'];
 	}
 
 	/**

apps/dav/tests/unit/CardDAV/AddressBookImplTest.php

@@ -53,7 +53,8 @@ class AddressBookImplTest extends TestCase {
 	}
 
 	public function testGetKey(): void {
-		$this->assertSame($this->addressBookInfo['id'],
+		$this->assertIsString($this->addressBookImpl->getKey());
+		$this->assertSame((string)$this->addressBookInfo['id'],
 			$this->addressBookImpl->getKey());
 	}