Don't send headers on passthrough

[joeybaker]

Prevents aggressive cache headers from being set when st won't serve the route.

[rvagg]

oh, good one, will merge when I get a moment to have a good look at this, unless @isaacs does first, thanks!

[joeybaker]

Thanks. I drifted a bit from the code style with the ternary operator and am calling .isDirectory() twice, but it seemed clean~ish to me?

FWIW, this caused a really subtle bug for us that took forever to track down.

[isaacs]

Landed on 2de1baa. Thanks!