* putSaver: detect edit conflicts to prevent clobbering, if possible
if the server supplies an ETag, we send it back when saving, allowing
the server to detect edit conflicts and respond with 412 (cf.
https://www.w3.org/1999/04/Editing/)
caveats:
* this only kicks in after the first save, as we don't have access to
the ETag when first loading the document
* there's no recovery mechanism (e.g. resetting `this.etag` in order to
force clobbering), other than manually reloading the document
* putSaver: retrieve ETag upon initialization for clobbering protection
this addresses one of the caveats from the previous commit
(2d75cb83af) - while theoretically prone
to a race condition, it seems unlikely that saving will be triggered
before the server responds
* putSaver: simplify URI extraction
this simplifies the approach introduced in
f51f6bf774, with the purpose of removing
the fragment identifier
* putSaver: localize error message
* putSaver: switch to built-in HTTP helper
in the process, fixed ETag assignment in `#save` method (was
`this.etag`, now `self.etag`) as well as a syntax error due to a missing
closing brace
* putSaver: consolidate URI handling
