fix(php): do not cast to string something that's already string

Author: simPod

modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache

@@ -340,7 +340,7 @@ class ObjectSerializer
         } elseif (is_bool($value)) {
             return $value ? 'true' : 'false';
         } else {
-            return (string) $value;
+            return $value;
         }
     }
 

samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php

@@ -349,7 +349,7 @@ public static function toString($value)
         } elseif (is_bool($value)) {
             return $value ? 'true' : 'false';
         } else {
-            return (string) $value;
+            return $value;
         }
     }
 

samples/client/petstore/php/psr-18/lib/ObjectSerializer.php

@@ -349,7 +349,7 @@ public static function toString($value)
         } elseif (is_bool($value)) {
             return $value ? 'true' : 'false';
         } else {
-            return (string) $value;
+            return $value;
         }
     }