Fix block syntax for buttons

If you use the block syntax, then the href is actually confusingly the variable named `name` as it is the first argument, rather than the one named `href`.

Unfortunately the tests didn't pick this up as the mock for `url_for` always returned the right variable, even if passed `nil`. Swapping this out for a minimal implementation of `url_for` avoids these false positives.

app/helpers/govuk_link_helper.rb

@@ -26,7 +26,7 @@ def govuk_button_to(name, href = nil, disabled: false, inverse: false, secondary
     button_text = build_text(name, visually_hidden_prefix: visually_hidden_prefix, visually_hidden_suffix: visually_hidden_suffix)
 
     if block_given?
-      button_to(href, **button_args, &block)
+      button_to(name, **button_args, &block)
     else
       button_to(button_text, href, **button_args)
     end
@@ -37,7 +37,7 @@ def govuk_button_link_to(name, href = nil, new_tab: false, disabled: false, inve
     button_text = build_text(name, visually_hidden_prefix: visually_hidden_prefix, visually_hidden_suffix: visually_hidden_suffix)
 
     if block_given?
-      link_to(href, **button_args, &block)
+      link_to(name, **button_args, &block)
     else
       link_to(button_text, href, **button_args)
     end

spec/helpers/govuk_link_helper_spec.rb

@@ -19,7 +19,9 @@
   include ActionView::Helpers::UrlHelper
 
   before do
-    allow(self).to receive(:url_for).with(any_args).and_return("/world")
+    def url_for(path)
+      return path
+    end
   end
 
   describe "govuk_link_to" do