Restore ruby 2.5 support

As far as I know, this is the only stdlib that has dropped ruby 2.5,
which hasn't yet reached its End of Life date.

The motivation for this is that there's no gemified version of
`net-http` that supports ruby 2.5, and that makes it hard for libraries
depending on `net-http` to properly declare the dependency, since the
only way is to drop support for ruby 2.5, which many maintainers will
not yet want to do.

The reason for wanting to declare the `net-http` dependency is mainly
that it helps bundler & rubygems with avoiding double loads of different
versions of `net-http` (the stdlib version and a higher gemified version).

As far as I can observe from commit history, the only reason for
dropping support for ruby 2.5 was that tests didn't pass, so I made them
pass.

net-http.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |spec|
   spec.summary       = %q{HTTP client api for Ruby.}
   spec.description   = %q{HTTP client api for Ruby.}
   spec.homepage      = "https://github.com/ruby/net-http"
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.6.0")
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.5.0")
   spec.licenses      = ["Ruby", "BSD-2-Clause"]
 
   spec.metadata["homepage_uri"] = spec.homepage

test/net/http/test_https.rb

@@ -45,7 +45,7 @@ def test_get
       assert_equal($test_net_http_data, res.body)
     }
     # TODO: OpenSSL 1.1.1h seems to yield only SERVER_CERT; need to check the incompatibility
-    certs.zip([CA_CERT, SERVER_CERT][-certs.size..]) do |actual, expected|
+    certs.zip([CA_CERT, SERVER_CERT][-certs.size..-1]) do |actual, expected|
       assert_equal(expected.to_der, actual.to_der)
     end
   rescue SystemCallError
@@ -66,7 +66,7 @@ def test_get_SNI
       assert_equal($test_net_http_data, res.body)
     }
     # TODO: OpenSSL 1.1.1h seems to yield only SERVER_CERT; need to check the incompatibility
-    certs.zip([CA_CERT, SERVER_CERT][-certs.size..]) do |actual, expected|
+    certs.zip([CA_CERT, SERVER_CERT][-certs.size..-1]) do |actual, expected|
       assert_equal(expected.to_der, actual.to_der)
     end
   end