replace_header(): New method given by Skip Montanaro in SF patch #601959.  Modified slightly by Barry (who added the KeyError in case the header is missing.
replace_header(): New method given by Skip Montanaro in SF patch
#601959.  Modified slightly by Barry (who added the KeyError in case
the header is missing.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator