Add list of allowed methods to HttpMethodNotAllowedException

When we know the allowed methods, add them to the exception message so
that it's easier to see what happened in error logs.

Slim/Exception/HttpMethodNotAllowedException.php

@@ -36,6 +36,7 @@ public function getAllowedMethods(): array
     public function setAllowedMethods(array $methods): HttpMethodNotAllowedException
     {
         $this->allowedMethods = $methods;
+        $this->message = 'Method not allowed. Must be one of: ' . implode(', ', $methods);
         return $this;
     }
 }

tests/Exception/HttpExceptionTest.php

@@ -43,8 +43,10 @@ public function testHttpNotAllowedExceptionGetAllowedMethods()
         $exception = new HttpMethodNotAllowedException($request);
         $exception->setAllowedMethods(['GET']);
         $this->assertEquals(['GET'], $exception->getAllowedMethods());
+        $this->assertEquals('Method not allowed. Must be one of: GET', $exception->getMessage());
 
         $exception = new HttpMethodNotAllowedException($request);
         $this->assertEquals([], $exception->getAllowedMethods());
+        $this->assertEquals('Method not allowed.', $exception->getMessage());
     }
 }