Add default return value for request data

Allow overloading of the default return value of NULL when
requesting request data.

Example:
  http://host/action?foo=bar&abc=123

  $app->request->get('foo')        -- [bar]
  $app->request->get('bar')        -- [NULL]
  $app->request->get('bar', false) -- [FALSE]

Change-Id: I41b9756b531eb06ee5117acc03ad3be5b500d4f6