Allow 'Host' header instead of 'host' and remember case across redirects

As of v2.46.0 (because of #1184) this crashes when a redirect occurs.
We should also preserve the case of the header name if the user
specifies 'Host', because lots of (broken) servers expect the header
name to be 'Host' and not 'host'.
Merge pull request #1211 from nylen/host-header-uppercase

Allow 'Host' header instead of 'host' and remember case across redirects