Support Unicode text in header values

[lini]

Hi!

I had some problems using this library on websites that send UTF-8 encoded values for some headers (e.g. non-English text), so I changed the encoding/decoding routines to use UTF8 encoding instead of ASCII. This change should have no conflicts with the current behavior, since UTF8 will act the same as ASCII for character codes < 127 (i.e. normal text). With the existing implementation, all Unicode characters are converted to ? in the header value.