Add CreateDavFile/CreateDavTextFile/CreateDavHtmlFile to create an empty file
(the upload endpoint rejects empty bodies) and UpdateDavFile to replace an
existing file in place via PUT /files/{id}/update.
The /upload endpoint creates a second file when the title already exists, so a
WebDAV overwrite must go through /update to keep a single file. uploadReader now
takes the HTTP method.
