Add dependency on net-http gem

As described in https://www.ruby-lang.org/en/news/2020/12/25/ruby-3-0-0-released/, Ruby
3.0.0 promoted net-http from stdlib. Adding this as a dependency avoids
"already initialized constant errors" if net-imap or some other gem is
included. See ruby/net-imap#16 (comment) for
more details.

faraday-net_http.gemspec

@@ -22,6 +22,12 @@ Gem::Specification.new do |spec|
   spec.files = Dir.glob('lib/**/*') + %w[README.md LICENSE.md]
   spec.require_paths = ['lib']
 
+  # net-http was promoted from stdlib to became a default gem in Ruby 3.0.0.
+  # This gem is only available for Ruby 2.6+.
+  if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('2.6.0')
+    spec.add_dependency 'net-http', '~> 0.1'
+  end
+
   spec.add_development_dependency 'faraday', '~> 1.0'
 
   spec.add_development_dependency 'bundler', '~> 2.0'