std.net.curl: Allow user to specify Content-Type in POST requests

Fixes http://d.puremagic.com/issues/show_bug.cgi?id=10911

I'm not sure setPostData fits into the overall API of std.net.curl.HTTP, but since postData is a @property, it's not possible to just add a default parameter to it. If anyone has better suggestions, please let me know. In any case, the inability to specify Content-Type in POST requests makes std.net.curl.HTTP pretty much useless for me.