Fix invalid multibyte escape error.

To fix these errors:

```
/Users/Mac/.rvm/gems/ruby-2.0.0-p247@quiz2/gems/signet-0.3.2/lib/signet.rb:22: invalid multibyte escape: /[\s\x21\x23-\x5B\x5D-\x7E\x80-\xFF]/
invalid multibyte escape: /\\[\s\x21-\x7E\x80-\xFF]/
invalid multibyte escape: /[\s\x21-\x26\x28-\x5B\x5D-\x7E\x80-\xFF]/
invalid multibyte escape: /\\[\s\x21-\x7E\x80-\xFF]/
```

I'm using Ruby 2.0.0-p247.