ocsigenserver: implement "Expect: 100-continue" support

Match the expect header case insensitively, and send a 100-continue header.
This makes the PUT query in a64f168cd57f4276cd1d6d698d18f290a6835a9c actually
useful.

The code was tested on top of ocsigenserver-2.2.0 for quite some time,
as shown by the original commit date.