[master] removed content type header

I suggest to remove the JSON header, because, in the current version, executing code snippets like in the following is not possible:

    curl -d 'x=data.frame(x=1,y=1)' https://public.opencpu.org/ocpu/library/base/R/identity

It is not allowed to submit non-JSON values for arguments (here: for 'x'). But one needs to pass a verbatim R expression for arbitrary code execution. See for example [this fiddle](http://jsfiddle.net/opencpu/v42v3/).