fix for detect_encoding returning a hash without :encoding key

[Holek]

this can happen for binary strings, or strings that appear as binary:

ruby-1.9.3-p125-perf :017 > "BEGIN:VCARD\n".detect_encoding
 => {:type=>:binary, :confidence=>100} 

[toch]

I've got the same error when trying to detect encoding on an empty string. I confirm that the @Holek's fix works. Thx.

[Holek]

This is probably also not the best workaround, as it does not return :binary as encoding, but nil. (At least it's not throwing an error)

[brianmario]

ah good catch thanks!