Allow stringeable objects to be serialized by StringType

Closes webonyx#302

Author: theofidry

src/Type/Definition/StringType.php

@@ -40,6 +40,9 @@ public function serialize($value)
         if ($value === null) {
             return 'null';
         }
+        if (is_object($value) && method_exists($value, '__toString')) {
+            return (string) $value;
+        }
         if (!is_scalar($value)) {
             throw new Error("String cannot represent non scalar value: " . Utils::printSafe($value));
         }

tests/Type/ScalarSerializationTest.php

@@ -150,6 +150,7 @@ public function testSerializesOutputStrings()
         $this->assertSame('true', $stringType->serialize(true));
         $this->assertSame('false', $stringType->serialize(false));
         $this->assertSame('null', $stringType->serialize(null));
+        $this->assertSame('2', $stringType->serialize(new ObjectIdStub(2)));
     }
 
     public function testSerializesOutputStringsCannotRepresentArray()