Fix #26 by running assertion when we're aren't testing for a body.

We're creating Nokogiri documents from HTML fragments, which means Nokogiri automatically
adds a body element which is unexpected.

Author: kaspth

lib/rails/dom/testing/assertions/selector_assertions.rb

@@ -165,6 +165,12 @@ def assert_select(*args, &block)
           @selected ||= nil
 
           selector = HTMLSelector.new(args, @selected) { nodeset document_root_element }
+
+          if selecting_no_body?(selector)
+            assert true
+            return
+          end
+
           selector.select.tap do |matches|
             assert_size_match!(matches.size, selector.tests, selector.selector, selector.message)
 
@@ -276,6 +282,12 @@ def assert_size_match!(size, equals, css_selector, message = nil)
             end
           end
 
+          def selecting_no_body?(html_selector)
+            # Nokogiri gives the document a body element. Which means we can't
+            # run an assertion expecting there to not be a body.
+            html_selector.selector == 'body' && html_selector.tests[:count] == 0
+          end
+
           def nest_selection(selection)
             # Set @selected to allow nested assert_select.
             # Can be nested several levels deep.