Content type fix

I've forked etest_http to add something that wasn't working. The scenario is when using ?perform_post macro. Basically you couldn't specify correctly 'content-type' provided in the body. It was being passed as "" to httpc regardless what you would send on the Headers. 

httpc erlang HTTP client takes the content-type as string as the third element of the tuple within the Request. In your 'master', it's always passed as empty string. My fix let the user provide the 'content-type' value as part of the proplists Headers (3rd argument of `etest_http:perform_request/5`.