Fix regexp to use case-insensitivity correctly

I noticed a faulty regexp in Sinatra's uri():
The regexp /[A-z]/ matches all alphabets, but also six non-alphabetic characters, as demonstrated by ("A".."z").to_a.join
=> "ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz"