Add net-http gem dependency to silence net-protocol warnings

[onurkucukkece]

Issue #279

After the gem update to v1.12.0, we started to get below warnings on net-protocol gem.

This PR adds net-http to gem dependencies for earlier versions of Ruby v3 to silence these warnings as suggested in ruby/net-imap#16

.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/net-protocol-0.1.3/lib/net/protocol.rb:68: warning: already initialized constant Net::ProtocRetryError
.rbenv/versions/2.7.5/lib/ruby/2.7.0/net/protocol.rb:66: warning: previous definition of ProtocRetryError was here
.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/net-protocol-0.1.3/lib/net/protocol.rb:208: warning: already initialized constant Net::BufferedIO::BUFSIZE
.rbenv/versions/2.7.5/lib/ruby/2.7.0/net/protocol.rb:206: warning: previous definition of BUFSIZE was here
.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/net-protocol-0.1.3/lib/net/protocol.rb:504: warning: already initialized constant Net::NetPrivate::Socket
.rbenv/versions/2.7.5/lib/ruby/2.7.0/net/protocol.rb:503: warning: previous definition of Socket was here

[fphilipe]

Conditional dependencies in gemspecs don't really work as expected. See also 5b2aa73#commitcomment-90641731.

I'll have to dig a bit deeper to understand what's going on here.

Closing this PR as it's not the proper fix. Thanks nonetheless for your effort 😍