Fix tests for status code change on Rack HEAD

Rack was recently [updated][1] with a deprecation for some status codes
that have been renamed (most notably, Unprocessable Entity was renamed
to Unprocessable Content). Since the deprecation was only added to
the `#status_code` method, this has caused test failures for some tests
that depend on the `SYMBOLS_TO_STATUS_CODE` hash.

This commit replaces the usage of `SYMBOLS_TO_STATUS_CODE` with
`#status_code` so that we get the deprecation message instead of a test
failure.

[1]: rack/rack@64ad26e

actionpack/lib/action_dispatch/testing/assertion_response.rb

@@ -36,7 +36,7 @@ def code_and_name
 
     private
       def code_from_name(name)
-        GENERIC_RESPONSE_CODES[name] || Rack::Utils::SYMBOL_TO_STATUS_CODE[name]
+        GENERIC_RESPONSE_CODES[name] || Rack::Utils.status_code(name)
       end
 
       def name_from_code(code)