Fix usage of __METHOD__ inside of closure

The value of __METHOD__ would be a description of the nearest function-like, i.e. `'{closure}'`

This was detected via static analysis (Phan)

src/Whoops/Exception/FrameCollection.php

@@ -61,7 +61,7 @@ public function map($callable)
 
             if (!$frame instanceof Frame) {
                 throw new UnexpectedValueException(
-                    "Callable to " . __METHOD__ . " must return a Frame object"
+                    "Callable to " . __CLASS__ . "::map must return a Frame object"
                 );
             }