This is a slightly improved v2 version.
It adds the option to escape colons and flag
characters (:}!) with a backslash if you want
them to be interpreted as literal characters
(ex: \!).
This means that literal backslashes must also
be escaped (\\). flags need to be escaped only
if they are the first character in the string.
Note that header fields still mustn't contain
a colon character.