Fix for complete URLs and additions to handle more cases of complete URLs

This started as a fix for when passed strings were already complete URLs, as is the case when you pass: request.build_absolute_uri in a template.  Prior to this fix, the resulting shortened URL would be malformed:

input: http://domain.com/resource/
output: http://domain.comhttp/domain.com/resource/

Works fine now.  Also, added support to properly detect complete URLs for schemes that use zero or one forward slash too. such as mailto: or tel: or bitcoin:

BTW, nicely designed code.  It was easy to jump into and the whole stringholder thing is nice.

Signed-off-by: Martin Koistinen <mkoistinen@gmail.com>