[bug] ProxyHeaders XFF parsing handles whitespace incorrectly

[adam-p]

Describe the bug

The current code looks like this:

		s := strings.Index(fwd, ", ")
		if s == -1 {
			s = len(fwd)
		}
		addr = fwd[:s]

As can be seen in this playground, it doesn't handle whitespace around the comma properly. (The playground isn't working consistently, I think because of back-end weirdness. But the code works.)

If the header is like 1.1.1.1,2.2.2.2, 3.3.3.3, the resulting addr will be 1.1.1.1,2.2.2.2. If the header is like 1.1.1.1 , 2.2.2.2, the resulting addr will be 1.1.1.1<space>.

RFC 2616 states that there can be "linear white space" (LWS; space or tab) around the values and commas. For example:

each separated by one or more commas (",") and OPTIONAL linear white space (LWS). This makes the usual form of lists very easy; a rule such as
( *LWS element *( *LWS "," *LWS element ))

And:

Except where noted otherwise, linear white space (LWS) can be included between any two adjacent words (token or quoted-string), and between adjacent words and separators, without changing the interpretation of a field.

I will submit a PR shortly.

Versions
go1.18
master HEAD

Steps to Reproduce

Code here: https://go.dev/play/p/uTO1eaTRsbx

Expected behavior

Proper handling of whitespace.