net/http: clean up checkIfModifiedSince and checkIfUnmodifiedSince
net/http: clean up checkIfModifiedSince and checkIfUnmodifiedSince

The comment in both functions referred to the wrong header and I made
the checks easier to read.

Change-Id: Ifb46729cee631a3305f557863818e3487b8eed71
Reviewed-on: https://go-review.googlesource.com/c/go/+/71753
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

akutz/go,cloudflare/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,akutz/go,golang/go,akutz/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,cloudflare/go,akutz/go,nawawi/go,cloudflare/go,akutz/go,golang/go,nawawi/go,cloudflare/go,akutz/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go