[BUG] Add "Vary" header only if value not already present

[gaston-haro]

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

Responses may end up with duplicate header key:value for "Vary:Accept-Encoding"

Expected Behavior

Adding the header value should only be done if it is not already present.

Steps To Reproduce

No response

Anything else?

handlers/compress.go

Line 95 in 9c61bd8

w.Header().Add("Vary", acceptEncoding)

[gaston-haro]

-		w.Header().Add("Vary", acceptEncoding)
+		if w.Header().Get("Vary") != acceptEncoding {
+			w.Header().Add("Vary", acceptEncoding)
+		}