Allows HttpClient to call https urls

Adds parameter "-k" to every way of calling curl, hence allowing
calling https URLs without checking for the validity of SSL
certificates.
While this makes it a little insecure, nothing else can be done
while keeping the HTTPClient API simple: openwrt does not have a
SSL certificates bundle
Advanced users concerned about security should call "curl" on
their own using Process, supplying parameters such as "--cacert"
Fixes #1860
@cmaglie