solves #383
fixes #383
* mu4e: update the contact-rewriting code a bit:

  - make the default nil rather than a dummy (identity) function, for
    speed
  - don't use 'canonical' but the 'rewrite' which is a bit more general
* Merge branch 'master' of github.com:djcb/mu
* bump version to 0.9.9.6pre3