don't raise warnings when exception is thrown

`array_map()` raises a warning when an exception is thrown inside the
callback (see https://bugs.php.net/bug.php?id=55416). To avoid these
warnings, `selectorToXPath()` is applied inside the loop.
Merge branch '2.3' into 2.5

* 2.3:
  [Routing] add tests for RequestContext
  [Routing] fix inconsistencies in RequestContext
  don't raise warnings when exception is thrown