implemented Connection#exhausted?

an exhausted connection can't service more requests, therefore is
unmatchable and can't enqueue more requests to the parser.

lib/httpx/connection.rb

@@ -80,6 +80,7 @@ module HTTPX
     def match?(uri, options)
       return false if @state == :closing || @state == :closed
 
+      return false if exhausted?
       (
         (
           @origins.include?(uri.origin) &&
@@ -95,6 +96,8 @@ module HTTPX
     def mergeable?(connection)
       return false if @state == :closing || @state == :closed || !@io
 
+      return false if exhausted?
+
       !(@io.addresses & connection.addresses).empty? && @options == connection.options
     end
 
@@ -213,6 +216,10 @@ module HTTPX
 
     private
 
+    def exhausted?
+      @parser && parser.exhausted?
+    end
+
     def consume
       catch(:called) do
         dread

lib/httpx/connection/http1.rb

@@ -31,6 +31,10 @@ module HTTPX
       emit(:close)
     end
 
+    def exhausted?
+      !@max_requests.positive?
+    end
+
     def empty?
       # this means that for every request there's an available
       # partial response, so there are no in-flight requests waiting.

lib/httpx/connection/http2.rb

@@ -35,11 +35,15 @@ module HTTPX
       @connection.state == :closed || @streams.empty?
     end
 
+    def exhausted?
+      @connection.active_stream_count >= @max_concurrent_requests
+    end
+
     def <<(data)
       @connection << data
     end
 
-    def send(request, **)
+    def send(request)
       if !@handshake_completed ||
          @streams.size >= @max_concurrent_requests
         @pending << request