fix: HTTP client

[noahlevenson]

This PR introduces a number of changes which fix HTTP client bugs, implement correct SSL/TLS semantics, and make our two underlying HTTP client libraries -- gun and httpc -- match one another's behavior.

Changes:

• gun now automatically follows 3xx redirects, matching httpc behavior
• new http_follow_redirects option toggles autoredirect behavior, works for both gun and httpc
• new gun_max_redirects option mitigates redirect attacks on our bespoke gun redirect follower (httpc handles redirects out of the box and does not expose a twiddleable param)
• deriving transport (TLS over TCP vs. plaintext TCP) from port and scheme is fixed
• URLs without explicit ports now work the way you expect
• TLS over arbitrary ports now works

Now, you should be able to do something like this:

hb_http_client:req(#{path => "/", method => <<"GET">>, peer => "http://nytimes.com", headers => #{}, body => <<>>}, #{http_client => gun}).

And it should just work. That request actually follows 2 redirects -- first, to the TLS encrypted https:// URL, and then to the www subdomain. So just for fun, if you twiddle the gun_max_redirects option, you can observe the client only follow the first redirect and return the 301:

hb_http_client:req(#{path => "/", method => <<"GET">>, peer => "http://nytimes.com", headers => #{}, body => <<>>}, #{http_client => gun, gun_max_redirects => 1}).

[noahlevenson]

Reference for #481

[noahlevenson]

Reference for #481