Directory methods fixes

[blootsvoets]

On our DAV server, two functions on directories failed:

1. check() returned False when passed dav.check('/path/to/mydir'), where mydir is a directory
2. is_dir() raised a MethodNotSupported error on files

Fixed these two issues by

1. removing trailing slashes from both the path and urn before comparing
2. looking for a resourcetype/collection directly, thus resources without a resource type will not fail but return false.

Could you please consider these changes for a next release?