check for lt returned by GetCachedInstance being null

Under some very weird circunstances that I haven't been able to reproduce but I'm seeing the logs for, lt is null. 
Since GetCachedInstance returns a null in some instances I would also argue this check should be in place anyways to avoid the null reference exception when the conditions for that null return occur.
Merge pull request #75 from raulvejar/patch-1

check for lt returned by GetCachedInstance being null
Proposed fix for #73. EarlyUrlLocalization redirection on unlocalized URLs now only happens with GET and HEAD methods. For other methods such as POST, redirection is avoided; instead the PrincipalApplicationLanguage for the request is inferred and the request handled without informing the browser (via the redirect) of the actual URL, not seen as an issue given that the URLs of POST methods etc. are not typically shown to the user.