Barry raised reasonable objections to the macro name \mimeheader, so

we'll simply revert to \mailheader since there's no other good name.

Doc/lib/libcookie.tex

@@ -19,7 +19,7 @@ specs.  As a result, the parsing rules used are a bit less strict.
 
 \begin{excdesc}{CookieError}
 Exception failing because of \rfc{2109} invalidity: incorrect
-attributes, incorrect \mimeheader{Set-Cookie} header, etc.
+attributes, incorrect \mailheader{Set-Cookie} header, etc.
 \end{excdesc}
 
 \begin{classdesc}{BaseCookie}{\optional{input}}