Fixed test for assert_welcome

 - Test was wrongly using assert with assignment statement.

railties/test/isolation/abstract_unit.rb

@@ -69,7 +69,8 @@ def get(path)
 
     def assert_welcome(resp)
       assert_equal 200, resp[0]
-      assert resp[1]["Content-Type"] = "text/html"
+      assert_match 'text/html', resp[1]["Content-Type"]
+      assert_match 'charset=utf-8', resp[1]["Content-Type"]
       assert extract_body(resp).match(/Welcome aboard/)
     end