Changes in commit 5697359 broke the around modifier when used as a
getter. Rather than revert, this commit cleans up the logical cases the
modifier has to deal with:

  * If called as a "getter", the modifier is only passed two params.
    Check for that case and return the result of the original method.

  * If there is no serializer defined, encode the content and call the
    original method.

  * Otherwise serialize any content, then call the original method.

Also reverts the changes to the before_serializer counts in t/hooks.t
from the same commit.

Adds a test for an app with a serializer defined that has routes that
  * return no conent ( /nothing )
  * return content to be serialized (all the others)
verifying that the before_serializer hook can change the response content
in both cases. See #1053.

If a serializer is defined, before_serializer hooks may change the
response content, including the "default" of an empty string if no route
returned any content for the request.

Set the content to the empty string if the response has no defined
content allowing any hooks to betriggered before constructing the
PSGI response.