fix(middleware): match WWW redirects case-insensitively

[lowgame]

Reproduction

The WWW redirect helpers compare the www. host label case-sensitively. Since DNS hostnames are case-insensitive, an HTTPS request for WWW.labstack.com is already on the WWW host, but HTTPSWWWRedirect currently redirects it to https://www.WWW.labstack.com/. Conversely, the non-WWW helpers fail to remove that uppercase label.

The added table cases reproduce this behavior for all four WWW/non-WWW redirect variants; all four fail against the unchanged implementation.

Root cause

The helpers use case-sensitive strings.HasPrefix and strings.TrimPrefix checks for the hostname label.

Solution

Recognize the bounded www. prefix with strings.EqualFold, and remove the matched four-byte label while preserving the rest of the original host.

Tests

• Before the production fix: go test ./middleware -run 'TestRedirect(HTTPSWWW|HTTPSNonWWW|WWW|NonWWW)Redirect$' -count=1 — failed in all four new WWW.labstack.com cases.
• After the fix: go test ./middleware -run 'TestRedirect(HTTPSWWW|HTTPSNonWWW|WWW|NonWWW)Redirect$' -count=1 — passed.
• go test -race ./... -count=1 — passed (3 packages).
• go vet ./... — passed.
• make lint — passed (staticcheck and golint).