! 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.

spray-testkit/src/main/scala/spray/testkit/RouteTest.scala

@@ -91,15 +91,16 @@ trait RouteTest extends RequestBuilding with RouteResultComponent {
       type Out = HttpRequest
       def apply(request: HttpRequest, f: HttpRequest ⇒ HttpRequest) = f(request)
     }
-    implicit def injectIntoRoute(implicit timeout: RouteTestTimeout, settings: RoutingSettings, log: LoggingContext) =
+    implicit def injectIntoRoute(implicit timeout: RouteTestTimeout, settings: RoutingSettings,
+                                 log: LoggingContext, eh: ExceptionHandler) =
       new TildeArrow[RequestContext, Unit] {
         type Out = RouteResult
         def apply(request: HttpRequest, route: Route) = {
           val routeResult = new RouteResult(timeout.duration)
           val effectiveRequest =
             try request.withEffectiveUri(securedConnection = false)
             catch { case NonFatal(_) ⇒ request }
-          ExecutionDirectives.handleExceptions(ExceptionHandler.default)(route) {
+          ExecutionDirectives.handleExceptions(eh orElse ExceptionHandler.default)(route) {
             RequestContext(
               request = effectiveRequest,
               responder = routeResult.handler,