Create a browseable filesystem handler (like hFileSystem) but make all files
writeable by a PUT request. Files that do not exists will be created as long
as the directory in which they will be created exists.
This handler takes a FilePath and will try to store the entire request body in
that file. When the request body could for some reason not be fetch a
BadRequest error response will be created. When an IO error occurs the
hIOError function is used to setup an apropriate response.