Fix in RelativePathAdapter

Sometimes `totalDirectoriesUp - (relativePathIsInSourceDirectory ? 0 : _directoriesDown.Length)` can be == -1, and `Enumerable.Range()` throws exception
ResolveAppRelativePathToFileSystem can be broken when app isn't in root folder on server

Fix is required for processing css imports `Bundle.Css().Add(...).ProcessImports()...`
Merge branch 'patch-3' of https://github.com/kubelsmieci/SquishIt
fix tests with mocked http context for path resolution