Fixed return type for outputAs

This is a very minor change. Return type isn't mixed. _outputAs is declared as int, and all exit points should have a return statement when phpdoc declares a return type.

src/Console/ConsoleOutput.php

@@ -301,7 +301,7 @@ class ConsoleOutput
      * Get/Set the output type to use. The output type how formatting tags are treated.
      *
      * @param int|null $type The output type to use. Should be one of the class constants.
-     * @return mixed Either null or the value if getting.
+     * @return int|null Either null or the value if getting.
      */
     public function outputAs($type = null)
     {
@@ -309,6 +309,7 @@ class ConsoleOutput
             return $this->_outputAs;
         }
         $this->_outputAs = $type;
+        return null;
     }
 
     /**