replace_header has a different semantic than del-and-set (replace_header leaves the header in the same location in the list, rather than appending it to the end...that's it's purpose).  If replace_header is throwing a key error, then I guess we need a look-before-you-leap if statement.

And a test :)

Note that a correct fix would preserve the broken input email, but since we're obviously already doing munging I'm fine with just making this work for now.