Ensure URLs use ASCII encoding

Ruby 2.0 now defaults to UTF-8 encoding which results in this message when you run rspec:

    Failures:
    
      1) GoogleSafeBrowsing::Canonicalize should pass Units tests provided by Google
         Failure/Error: GoogleSafeBrowsing::Canonicalize.url("http://\x01\x80.com/").should == "http://%01%80.com/";
         ArgumentError:
           invalid byte sequence in UTF-8
         # ./lib/google_safe_browsing/canonicalize.rb:20:in `gsub!'
         # ./lib/google_safe_browsing/canonicalize.rb:20:in `url'
         # ./spec/canonicalize_spec.rb:28:in `block (2 levels) in <top (required)>'

As the URLs were encoded in ASCII and ASCII-8BIT before, forcing the encoding to ASCII-8BIT resolves the issue.