MINOR: checks: Make matching on HTTP headers for expect rules less obscure

MINOR: checks: Make matching on HTTP headers for expect rules less obscure

A default statement in the switch testing the header name has been added to be
sure it is impossible to eval the value pattern on an uninitialized header
value. It should never happen of course. But this way, it is explicit.

And a comment has been added to make clear that ctx.value is always defined when
it is evaluated.

This patch fixes the issue #619.