allow http/1.0 to work with mixed-case Keep-Alive

[gregors]

tldr; Is that certain clients (in this case apache bench) will send http/1.0 requests with "Keep-Alive" and not "keep-alive" and we currently don't respect that.

I noticed this issue while comparing ab to other servers which seem to handle this correctly. I originally asked about this behavior in the elixir forums

After taking a look and doing some debugging I saw that we fail to lower-case the connection header prior to comparison. Instead of incurring the penalty of lower-casing the entire header I'm assuming the mixed-case pattern match is a performant solution? Strictly speaking we probably should lower-case it to be "compliant"

I did look at the get_connection_header_keys/1 function but it didn't really look like a good fit since it returns that list. Happy to hear feedback and change anything.

[mtrudel]

Thanks for the PR!

This is good for now. I'm planning on a pretty major overhaul of header processing after #297 et al land, and I may revisit it then.