! testkit: enable custom ExceptionHandlers in routing tests

Up to now the `~>` operator injecting a request into a test route always wrapped the route execution with the default ExceptionHandler thereby ignoring potentially existing custom ExceptionHandlers. This is fixed with this patch.
 The change to the public API consists of one additional member to the implicit `TildeArrow.injectIntoRoute` method, which should rarely be called directly. In most cases this change therefore shouldn't break anything.
= routing: fix `cache` directive erroneously wrapping exceptions bubbled up from its inner route, closes #361
Merge pull request #362 from spray/ticket/361

#361: fix `cache` directive erroneously wrapping exceptions bubbled up from its inner route