Add a test case for the previous commit.

The commit said it fixed a problem with headers >200 characters
long. But examination of the code suggests that it was a header of
exactly 200 characters long that caused the problem. So we add a test
case for that here.

Before the fix in the previous commit, valgrind would detect many
errors when replying to the message created with this test case. After
that commit, those errors are gone.