fix: Decode field names and filenames correctly

[defnull]

See #60

Question 1: The new content_disposition_[un]quote() functions have a really long name. Any better ideas? I wanted to distinguish those from the old header_[un]quote() functions.

~Question 2: We don't use the old quoting functions anymore, but others do. Keep them?~ Answer: Yes we use the old functions for all other headers (e.g. content-type). Do not deprecate them.