I'm have the same issue. Temporarily, I'm using this workaround found in http://support.appharbor.com/kb/getting-started/managing-environments.
<add key="Environment" value="Debug"/>
Appharbor will replace the value="Debug" to the environment that was set in AppHarbor https://appharbor.com/applications/{appname}/edit
.
Instead of checking for HttpContext.IsDebugingEnabled, I'll check for
var isInDebugMode = ConfigurationManager.AppSettings["Environment"].Equals("Debug", StringComparison.InvariantCultureIgnoreCase));