Make frame finder a selector

lib/capybara/selector.rb

@@ -381,3 +381,11 @@ Capybara.add_selector(:table) do
     xpath
   end
 end
+
+Capybara.add_selector(:frame) do
+  xpath do |locator|
+    xpath = XPath.descendant(:iframe)
+    xpath = xpath[XPath.attr(:id).equals(locator.to_s) | XPath.attr(:name).equals(locator)] unless locator.nil?
+    xpath
+  end
+end

lib/capybara/session.rb

@@ -345,9 +345,9 @@ module Capybara
       when Capybara::Node::Element
         locator
       when String
-        find(:xpath, XPath.descendant(:iframe)[XPath.attr(:id).equals(locator) | XPath.attr(:name).equals(locator)].to_xpath(:exact))
+        find(:frame, locator)
       when Integer
-        all(:css, 'iframe', minimum: locator+1)[locator]
+        all(:frame, minimum: locator+1)[locator]
       else
         raise ArgumentError
       end