HttpMethod method() {
        final HttpMethod method = (HttpMethod) cache.get(HttpHeaderNames.METHOD);
        if (method != null) {
            return method;
        }

        final String methodStr = get(HttpHeaderNames.METHOD);
        checkState(methodStr != null, ":method header does not exist.");
        final HttpMethod parsed = HttpMethod.isSupported(methodStr) ? HttpMethod.valueOf(methodStr)
                                                                    : HttpMethod.UNKNOWN;
        cache.put(HttpHeaderNames.METHOD, parsed);
        return parsed;
    }
Adds the specified {@code setCookies} with {@link HttpHeaderNames#SET_COOKIE}.
core/src/main/java/com/linecorp/armeria/common/HttpHeadersBase.java