Fixed ext storage webdav path encoding

- Some WebDAV servers like lighttpd need paths in URLs to be properly
encoded
- Added error log output when curl connection failed
- Added check for 'resourcetype' in case the WebDAV server doesn't
  support/return it
- Fixed touch() to return false if the server doesn't implement
  PROPPATCH
- Added optional delay in WebDAV unit tests to use when testing against
  lighttpd's WebDAV
- Fixes #6831 

I ran the unit tests against ownCloud and lighttpd.
The only failing ones with lighttpd are related to recursive delete which don't seem to be supported by lighttpd (we can add this in a separate issue/PR)

@bourgeoa if you like you can test with this patch