Log errors returned from OmniSharpServer to the debug log when debugging
Fix curl headers being erroneous on windows

I tested with a win7 virtual machine.
Here's how:
- add a -v flag to omnisharp--get-curl-command-windows-with-tmp-file
- (setq omnisharp-debug t)
- evaluate them before trying :P

curl -v reports the request headers. Now that errors in the request are
shown (in 70e76db) we can see the server responds with an html page
describing an "invalid request header"