How can I generate an n-character pseudo random string containing only `A-Z`, `0-9` like [SecureRandom.base64](http://ruby-doc.org/stdlib-1.9.2/libdoc/securerandom/rdoc/SecureRandom.html) without `"+"`, `"/"`, and `"="`? For example:

```
(0..n).map {(('1'..'9').to_a + ('A'..'Z').to_a)[rand(36)]}.join

```