When the Bundle mechanism caches bundle files into the
ir.attachment table, it can sometimes cache an empty
resource file (For example, if a less/saas compiled file
results in an empty CSS file) for the bundle URL.
The appropriate behavior for _serve_attachment()
when the browser loads that bundle URL is to return
an empty file (which is the correct content), instead of
redirecting again to the same URL, triggering a loop.
In addition, this commit removes the special case for
returning 204 No Content. This HTTP status code is not
really meant for GET requests - returning an empty file
with a 304 or 200 code is more appropriate and allows
for normal browser caching.