passing fallback protocol as option; this will allow to override, specially in the case when user knows that server can do unencrypted http/2

lib/httpx/io.rb

@@ -10,6 +10,7 @@ module HTTPX
     attr_reader :ip, :port, :uri
 
     def initialize(uri, options)
+      @fallback_protocol = options.fallback_protocol
       @connected = false
       @uri = uri
       @ip = TCPSocket.getaddress(@uri.host) 
@@ -32,7 +33,7 @@ module HTTPX
     end
 
     def protocol
-      "http/1.1"
+      @fallback_protocol 
     end
 
     def connect

lib/httpx/options.rb

@@ -43,6 +43,7 @@ module HTTPX
       defaults = {
         :proxy                    => {},
         :ssl                      => { alpn_protocols: %w[h2 http/1.1] },
+        :fallback_protocol        => "http/1.1", 
         :timeout                  => Timeout.by(:per_operation),
         :headers                  => {},
         :cookies                  => {},
@@ -94,7 +95,7 @@ module HTTPX
       params form json body
       proxy follow ssl max_retries
       request_class response_class headers_class response_body_class
-      io
+      io fallback_protocol
     ].each do |method_name|
       def_option(method_name)
     end

test/options_test.rb

@@ -67,6 +67,7 @@ class OptionsSpec < Minitest::Test
       :form               => {:bar => "bar"},
       :timeout            => Timeout::PerOperation.new,
       :ssl                => {:foo => "bar", :alpn_protocols => %w[h2 http/1.1] },
+      :fallback_protocol  => "http/1.1",
       :headers            => {"Foo" => "foo", "Accept" => "xml", "Bar" => "bar"},
       :proxy              => {:proxy_address => "127.0.0.1", :proxy_port => 8080},
       :cookies            => {},