Test that for X-Served-By is set by Edge

We check for the Fastly format of cache-server1234-LOC, which is going
to change between providers, but this at least starts that documentation
going.

Also, check for existance of this header.
Hmm, appears Go does not support anything other than simple types being marked constant with `const`: 'There are boolean constants, rune constants, integer constants, floating-point constants, complex constants, and string constants. Rune, integer, floating-point, and complex constants are collectively called numeric constants.'