Removed the explicit accept header in service client post call

Not sure what I was thinking when I did this the first time. I did have to make the service client accept headers on post requests to get things to work, but that provides more flexibility than having an explicit accept header parameter.

The original issue was that, using the Akka service client (which leverages the standard service client), I was unable to set the accept header to the appropriate values (application/json). Instead, it was always defaulting to application/xml. Ew.