Doesn't fully abide to RFC 7232

[raffi-semerciyan]

When using this library with some CDN providers, they would ignore the generated ETAG header due to the fact that the ETAG hash is not surrounded by double quotes as specified by RFC 7232.

Example of what this library is generating:
Etag: 2406314-3c0e9f103ba4e30b32ef737d0f83d8cee85e9fba

Example of what is expected according to RFC 7232:
Etag: "2406314-3c0e9f103ba4e30b32ef737d0f83d8cee85e9fba"

[drsybren]

FYI, there's more of RFC 7232 that's being ignored: go-http-utils/fresh#1