Phoenix 0.7.0 upgrade / Accept Header Bugfix

Fixed bug where default accept headers :[] result in always failing connection. 

Instead am using a set of defaults that were originally used for ACL on preflight as the default accept-headers for CORS request.

Updated Tests to account for this.

Bumped version to 0.6.0 to account for phoenix 0.7.0 release and plug 0.9.0 release ( plug's must explicitly halt chain )