FIX: Do not cache generated etag and last-modified date

When the etag or last-modified date of a resoure changes, e.g. due to
a PUT or POST request the upated value would not be generate but the
cached value would be sent.
Support customer message for default handlers

* Rename if-none-match to if-none-match? to be consistent
* Support keywords as resource functions
Add collection example
Improve get-put-example "thing-resource"

* Store and return content-type
* Send last modified date
Extract parse-source-definitions to parse decision graph