[datafeed parser] only update an entity attributes when needed. Closes #1989142 This may avoid error with some entities which have immutable attributes and don't check something actually changed. Also, avoid modifying the cached item representation, this may cause pb when an item is seen several times during an import.

doc/book usr/share/doc/cubicweb-documentation
doc/html usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc