class Mail::Field

evaluation.
treated in its unfolded form for further syntactic and semantic
that is immediately followed by WSP. Each header field should be
"unfolding". Unfolding is accomplished by simply removing any CRLF
of a header field to its single line representation is called
The process of moving from this folded multiple-line representation

2.2.3. Long Header Fields

def unfold(string)
  string.gsub(Constants::UNFOLD_WS, '\1')
end