#909 case insentive requests for images

[alfredmeier]

Our server recognized, that your tool tries to fetch a wrong image url.

We send emails with case-sensitive tokens to our users. But your tool tests the url with lowercase chars only.

Example:

In the email: http://example.com/emails/urls/TokeNaBcD

Your tool called:
"GET /emails/urls/tokenabcd HTTP/1.1" 404 1480 "-" "SpamBayes/1.1a3+ (Image)"