Don't override port if it's explicitly specified

lib/capybara/session.rb

@@ -247,15 +247,16 @@ module Capybara
       raise_server_error!
       @touched = true
 
-      visit_uri = URI.parse(visit_uri.to_s)
+      visit_uri = ::Addressable::URI.parse(visit_uri.to_s)
 
       uri_base = if @server
-        visit_uri.port = @server.port if config.always_include_port && (visit_uri.port == visit_uri.default_port)
-        URI.parse(config.app_host || "http://#{@server.host}:#{@server.port}")
+        ::Addressable::URI.parse(config.app_host || "http://#{@server.host}:#{@server.port}")
       else
-        config.app_host && URI.parse(config.app_host)
+        config.app_host && ::Addressable::URI.parse(config.app_host)
       end
 
+      uri_base.port ||= @server.port if @server && config.always_include_port
+
       # TODO - this is only for compatability with previous 2.x behavior that concatenated
       # Capybara.app_host and a "relative" path - Consider removing in 3.0
       # @abotalov brought up a good point about this behavior potentially being useful to people
@@ -264,7 +265,9 @@ module Capybara
         visit_uri.path = uri_base.path + visit_uri.path
       end
 
-      visit_uri = uri_base.merge(visit_uri) unless uri_base.nil?
+      if uri_base && [nil, 'http', 'https'].include?(visit_uri.scheme)
+        visit_uri = uri_base.merge(visit_uri.to_hash.delete_if { |k,v| v.nil? })
+      end
 
       driver.visit(visit_uri.to_s)
     end

lib/capybara/spec/session/visit_spec.rb

@@ -63,6 +63,23 @@ Capybara::SpecHelper.spec '#visit' do
       expect(URI.parse(@session.current_url).port).to eq(root_uri.port)
       expect(@session).to have_content('Another World')
     end
+
+    it "should add the server port to a visited url if no port specified", requires: [:server] do
+      expect(@session.driver).to receive(:visit).with("http://www.example.com:#{@session.server.port}")
+      @session.visit("http://www.example.com")
+    end
+
+    it "should not override the visit specified port even if default for scheme", requires: [:server] do
+      expect(@session.driver).to receive(:visit).with("http://www.example.com:80")
+      @session.visit('http://www.example.com:80')
+    end
+
+    it "should give preference to app_host port if specified", requires: [:server] do
+      Capybara.app_host = "http://www.example.com:6666"
+      expect(@session.driver).to receive(:visit).with("http://www.example.com:6666/random")
+      @session.visit('/random')
+    end
+
   end
 
   context "without a server", requires: [:server] do