immprove h2c to not misuse max_concurrrent_requests

the h2c was relying heavily on rewriting connection options to only allow the first request to upgrade; this changes by instead changing the parser on first incoming request, so that if it's upgrade and contains the header, blocks the remainder until the upgrade is successful or not, and if it's not reverts the max concurrent requests anyway

lib/httpx/connection/http1.rb

@@ -12,6 +12,8 @@ module HTTPX
 
     attr_reader :pending, :requests
 
+    attr_accessor :max_concurrent_requests
+
     def initialize(buffer, options)
       @options = Options.new(options)
       @max_concurrent_requests = @options.max_concurrent_requests || MAX_REQUESTS

lib/httpx/plugins/h2c.rb

@@ -73,20 +73,34 @@ module HTTPX
             @inflight -= prev_parser.requests.size
           end
 
-          parser_options = @options.merge(max_concurrent_requests: request.options.max_concurrent_requests)
-          @parser = H2CParser.new(@write_buffer, parser_options)
+          @parser = H2CParser.new(@write_buffer, @options)
           set_parser_callbacks(@parser)
           @inflight += 1
           @parser.upgrade(request, response)
           @upgrade_protocol = "h2c"
 
-          @options = @options.merge(max_concurrent_requests: nil)
-
           prev_parser.requests.each do |req|
             req.transition(:idle)
             send(req)
           end
         end
+
+        private
+
+        def send_request_to_parser(request)
+          super
+
+          return unless request.headers["upgrade"] == "h2c" && parser.is_a?(Connection::HTTP1)
+
+          max_concurrent_requests = parser.max_concurrent_requests
+
+          return if max_concurrent_requests == 1
+
+          parser.max_concurrent_requests = 1
+          request.once(:response) do
+            parser.max_concurrent_requests = max_concurrent_requests
+          end
+        end
       end
     end
     register_plugin(:h2c, H2C)

sig/connection/http1.rbs

@@ -10,8 +10,9 @@ module HTTPX
     attr_reader pending: Array[Request]
     attr_reader requests: Array[Request]
 
+    attr_accessor max_concurrent_requests: Integer
+
     @options: Options
-    @max_concurrent_requests: Integer
     @max_requests: Integer
     @parser: Parser::HTTP1
     @buffer: Buffer