Answer HEAD requests and give 405 a plain message (closes #111)

Every route was registered for GET only, so HEAD answered 405, and on
the JSON routes that 405 said "Internal server error". chi's GetHead
now serves HEAD with the GET handler. The /api/v1 and /ip route groups
answer a path they lack with a JSON 404 "Not found", and another method
with a JSON 405 "Method not allowed" whose Allow header names GET and
HEAD. JSONValidationMiddleware gives a 4xx sent without a JSON body the
status's standard text instead of "Internal server error".

Model: opus-5-5

internal/server/middleware.go

@@ -142,8 +142,9 @@ func deadlinePassed(ctx context.Context) bool {
 //
 // If a response is not valid JSON or is empty when JSON is expected, the middleware
 // returns a properly formatted JSON error response. If the request's time limit
-// has passed, it answers 504 with the error message "Request timeout". For other
-// errors, it returns "Internal server error" with status 500 if the original
+// has passed, it answers 504 with the error message "Request timeout". A 4xx
+// keeps its status, with the status's standard text as the message. For other
+// errors, it returns "Internal server error", with status 500 if the original
 // status was 200.
 //
 // Non-JSON responses are passed through unchanged.
@@ -203,8 +204,13 @@ func JSONValidationMiddleware(next http.Handler) http.Handler {
 		w.WriteHeader(statusCode)
 
 		errorMsg := internalErrorMessage
-		if statusCode == http.StatusGatewayTimeout {
+
+		switch {
+		case statusCode == http.StatusGatewayTimeout:
 			errorMsg = requestTimeoutMessage
+		case statusCode >= http.StatusBadRequest &&
+			statusCode < http.StatusInternalServerError:
+			errorMsg = http.StatusText(statusCode)
 		}
 
 		response := map[string]any{