Add HTTP client features table

[guoye-zhang]

Please review if these entries are accurate and suggest missing features

[czechboy0]

I was actually thinking that we'd remove the concrete clients and just have a Required vs optional column.

[guoye-zhang]

My opinion is that nothing should be required unless all 3 clients listed here support it, and even some features supported by all 3 might not be required (e.g. auto-redirect). I understand that this is not an opinion shared by everybody, so I thought that facts are more useful to document at this moment.

I could add a column with requirements, but I think none of them is required. We could allow a reverse HTTP client that supports none of the features to conform to the protocol.