gwi-mmuths and Synchro Add flag for using copiedHeaderFields and config for using extra head…
…ers in DKIM signature (#1468)

* Changed in DKIM signature

- Add flag for using copiedHeaderFields
- config for using extra headers in DKIM signature

* Cleanups after Review

- Don't use masked ampersands in url
- Remove test related artifacts after run

* Complete docs
Latest commit 5c9d3c5 Jun 26, 2018