[FIX] web: _serve_attachment should not trigger redirect loops

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.

openerp/addons/base/ir/ir_http.py

@@ -109,11 +109,9 @@ class ir_http(osv.AbstractModel):
             datas = attach[0]['datas'] or ''
             name = attach[0]['name']
 
-            if not datas:
-                if name.startswith(('http://', 'https://', '/')):
-                    return werkzeug.utils.redirect(name, 301)
-                else:
-                    return werkzeug.wrappers.Response(status=204)     # NO CONTENT
+            if (not datas and name != request.httprequest.path and
+                    name.startswith(('http://', 'https://', '/'))):
+                return werkzeug.utils.redirect(name, 301)
 
             response = werkzeug.wrappers.Response()
             server_format = openerp.tools.misc.DEFAULT_SERVER_DATETIME_FORMAT