allow http headers to be set to support auto-translate

In order for MailChimp to utilize auto-translate (for example sending a confirmation email in German), it requires the Accept-Language header to be sent.  Allow headers to be specified in the params
added test for passing in http headers params to api
added additional test for http headers
Merge pull request #67 from digitalmoksha/master

allow http headers to be set to support auto-translate