Merge pull request #4489 from murgatroid99/ruby_explicit_insecure

src/ruby/ext/grpc/rb_channel.c

src/ruby/lib/grpc/generic/client_stub.rb

src/ruby/lib/grpc/generic/service.rb

src/ruby/pb/test/client.rb

src/ruby/spec/call_spec.rb

src/ruby/spec/channel_spec.rb

src/ruby/spec/client_server_spec.rb

src/ruby/spec/generic/active_call_spec.rb

src/ruby/spec/generic/client_stub_spec.rb

src/ruby/spec/generic/rpc_server_spec.rb

src/ruby/spec/generic/service_spec.rb

src/ruby/spec/pb/health/checker_spec.rb