Remove Content-Type and Content-Length from folder descriptions

As @binbasti already mentioned on IRC, the Content-Length for folders should not be included in the directory listings.

In order to determine the content length for a directory, one would need to drill down through all its sub-directories and determine the size of every document. So in the worst case, getting the directory listing for the storage root would require to get the content length for every single document in the storage.

As the Content-Type for folders is always 'application/json', it doesn't yield any benefit to include it. It just increases the size of the listing.