Fixes infinite recursion issue when re-using XmlHttpRequest objects

Fix for stack overflows and out of memory errors from JS when
miniprofiler is used in conjunction with other 3rd party libraries like
Angular as they re-use the XmlHttpRequest.

Fix was originally made by jcoutch but never made it to the master
branch :

https://github.com/SamSaffron/MiniProfiler/pull/195