Make sure the paths passed to visit are prefixed with slashes (closes #249)

For Selenium and Celerity.

lib/capybara/driver/celerity_driver.rb

@@ -158,6 +158,7 @@ class Capybara::Driver::Celerity < Capybara::Driver::Base
 private
 
   def url(path)
+    path = "/#{path}" unless path =~ /^\//
     rack_server.url(path)
   end
 

lib/capybara/driver/selenium_driver.rb

@@ -155,6 +155,7 @@ class Capybara::Driver::Selenium < Capybara::Driver::Base
 private
 
   def url(path)
+    path = "/#{path}" unless path =~ /^\//
     rack_server.url(path)
   end
 

lib/capybara/spec/driver.rb

@@ -14,6 +14,11 @@ shared_examples_for 'driver' do
       @driver.visit('/foo')
       @driver.current_url.should include('/foo')
     end
+
+    it "should prefix paths with slashes to make sure the url is valid" do
+      @driver.visit('foo')
+      @driver.current_url.should include('/foo')
+    end
   end
 
   describe '#body' do