Fix deprecation notice for ReflectionType::__toString() in php 7.4

Add a method that does the exact same thing, as a simple workaround.

Fixes #23

lib/Dispatcher.php

@@ -124,7 +124,7 @@ public function dispatch($msg)
                         // Does the parameter have a type hint?
                         $param = $parameters[$position];
                         if ($param->hasType()) {
-                            $class = (string)$param->getType();
+                            $class = self::stringFromReflectionType($param->getType());
                             $value = $this->mapper->map($value, new $class());
                         }
                     } else if (is_array($value) && isset($docBlock)) {
@@ -160,4 +160,26 @@ public function dispatch($msg)
         $result = $obj->$fn(...$args);
         return $result;
     }
+
+    /**
+     * Converts the reflection type to a string representing that type.
+     * This avoids a deprecation notice in php 7.4, while behaving identically to __toString().
+     */
+    private static function stringFromReflectionType(
+        \ReflectionType $reflection_type = null
+    ) : string {
+        if (!$reflection_type) {
+            return '';
+        }
+        if ($reflection_type instanceof \ReflectionNamedType) {
+            $reflection_type_string = $reflection_type->getName();
+            if ($reflection_type->allowsNull()) {
+                return "?" . $reflection_type_string;
+            }
+            return $reflection_type_string;
+        }
+        // Unreachable in php 7.1+. Note that ReflectionType::__toString() is deprecated in php 7.4.
+        return (string)$reflection_type;
+    }
+
 }