Use custom selectors in within methods

This continues the centralization of XPath generation
and failure messages.

lib/capybara/selector.rb

@@ -100,6 +100,10 @@ Capybara.add_selector(:field) do
   xpath { |locator, options| XPath::HTML.field(locator) }
 end
 
+Capybara.add_selector(:fieldset) do
+  xpath { |locator, options| XPath::HTML.fieldset(locator) }
+end
+
 Capybara.add_selector(:link_or_button) do
   xpath { |locator, options| XPath::HTML.link_or_button(locator) }
   failure_message { |node, selector| "no link or button '#{selector.locator}' found" }

lib/capybara/session.rb

@@ -203,7 +203,7 @@ module Capybara
     # @param [String] locator    Id or legend of the fieldset
     #
     def within_fieldset(locator)
-      within :xpath, XPath::HTML.fieldset(locator) do
+      within :fieldset, locator do
         yield
       end
     end
@@ -215,7 +215,7 @@ module Capybara
     # @param [String] locator    Id or caption of the table
     #
     def within_table(locator)
-      within :xpath, XPath::HTML.table(locator) do
+      within :table, locator do
         yield
       end
     end