check that the database index is a number before selecting

[jonathanong]

closes #38

[mjackson]

Makes sense. Thanks!

[digitalcora]

@mjackson Will there be a point release that incorporates this fix? Just spent a good few hours banging my head against an "invalid DB index" error, because Heroku Redis provides URLs that look like:

redis://h:secretpassword@ec2-127-0-0-1.compute-1.amazonaws.com:6629

Note the "h", which I assume is included to avoid tripping up URL parsers that always expect a password to be accompanied by a username.

[amiel]

+1 for a point release; I need this too

Also, It seems like the standard for specifying the database is the first path segment instead of the http basic auth username (so redis://host/0 instead of redis://0:@host. What are your thoughts of using that instead?