Update heroku.md

Fix typo.
fix broken Built-in Initializer link
Mark ::MultiMime spec that is only failing on Travis CI (in certain rubies) as pending
Merge pull request #182 from gdi2290/patch-1

fix broken Built-in Initializer link in README.md
Merge pull request #169 from rtwomey/patch-1

Update heroku.md
Added tests custom header addition

* building off the work here: https://github.com/rumblelabs/asset_sync/pull/120