feat(compression): avoid setting Vary: Accept-Encoding when already set

[paolobarbolini]

Motivation

The compression layer always adds Vary: Accept-Encoding to the response headers, even if Vary: Accept-Encoding is already present.

Solution

Checks for the presence of the value before setting it. Ideally this would be done by properly parsing the Vary header, but given the current circumstances I've opted for just checking if the value case insensitively contains accept-encoding.

Closes #570.