added functional test to DoH resolver, and fixed the accept header (according to last RFC, it should only accept application/dns-message mime type)

lib/httpx/resolver/https.rb

@@ -9,6 +9,7 @@ module HTTPX
   class Resolver::HTTPS
     extend Forwardable
     include Resolver::ResolverMixin
+    using URIExtensions
 
     NAMESERVER = "https://1.1.1.1/dns-query"
 
@@ -38,7 +39,10 @@ module HTTPX
     end
 
     def <<(connection)
+      return if @uri.origin == connection.origin.to_s
+
       @uri_addresses ||= Resolv.getaddresses(@uri.host)
+
       if @uri_addresses.empty?
         ex = ResolveError.new("Can't resolve #{connection.origin.host}")
         ex.set_backtrace(caller)
@@ -176,8 +180,8 @@ module HTTPX
         payload = Resolver.encode_dns_query(hostname, type: RECORD_TYPES[type])
         request = rklass.new("POST", uri, @options.merge(body: [payload]))
         request.headers["content-type"] = "application/dns-message"
-        request.headers["accept"] = "application/dns-message"
       end
+      request.headers["accept"] = "application/dns-message"
       request.on(:response, &method(:on_response).curry[request])
       request.on(:promise, &method(:on_promise))
       request

test/https_test.rb

@@ -13,6 +13,7 @@ class HTTPSTest < Minitest::Test
   include IO
   include Timeouts
   include Errors
+  include Resolvers if ENV.key?("HTTPX_RESOLVER_URI")
   # TODO: uncomment as soon as nghttpx supports altsvc for HTTP/2
   # include AltSvc if ENV.key?("HTTPBIN_ALTSVC_HOST")
 

test/support/requests/resolvers.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+module Requests
+  module Resolvers
+    DOH_OPTIONS = { uri: ENV.fetch("HTTPX_RESOLVER_URI") }.freeze
+
+    SessionWithPool = Class.new(HTTPX::Session) do
+      def pool
+        @pool ||= HTTPX::Pool.new
+      end
+    end
+
+    def test_resolvers_doh
+      session = SessionWithPool.new
+      uri = build_uri("/get")
+      response = session.head(uri, resolver_class: :https, resolver_options: DOH_OPTIONS)
+      verify_status(response, 200)
+
+      resolvers = session.pool.instance_variable_get(:@resolvers)
+      assert resolvers.size == 1, "there should be one resolver"
+      resolver = resolvers.values.first
+      assert resolver.is_a?(HTTPX::Resolver::HTTPS)
+    end
+  end
+end