Set the Content-Type to application/json only if not already set.
Set the Content-Type to application/json only if not already set.

This allows exceptional uses of the JSON responseWriter to write
non JSON payload.
Example to come to explain this.
Issue: #63

thetooth/go-json-rest,hhy5861/go-json-rest,hiteshjoshi/go-json-rest,wingyplus/go-json-rest,AnuchitPrasertsang/go-json-rest,TuneLab/go-json-rest,beni55/go-json-rest,syrohei/go-json-rest,llaraujo/go-json-rest,zhengcongzhen/go-json-rest,snowsnail/go-json-rest,jacobxk/go-json-rest,observadorpt/go-json-rest,ant0ine/go-json-rest,thomasf/go-json-rest,unravelin/go-json-rest