Make sure application cache manifest files are not cached.

    private static final String NO_STORE = "no-store";

    private static final String NO_CACHE = "no-cache";

    private static final String MAX_AGE = "max-age";

    private static final String MANIFEST_MIME = "text/cache-manifest";

    private static long CACHE_THRESHOLD = 6 * 1024 * 1024;

    private static long CACHE_TRIM_AMOUNT = 2 * 1024 * 1024;

        // if the contentLength is already larger than CACHE_MAX_SIZE, skip it

        if (headers.getContentLength() > CACHE_MAX_SIZE) return null;

        // The HTML 5 spec, section 6.9.4, step 7.3 of the application cache

        // process states that HTTP caching rules are ignored for the

        // purposes of the application cache download process.

        // At this point we can't tell that if a file is part of this process,

        // except for the manifest, which has its own mimeType.

        // TODO: work out a way to distinguish all responses that are part of

        // the application download process and skip them.

        if (MANIFEST_MIME.equals(mimeType)) return null;

        // TODO: if authenticated or secure, return null

        CacheResult ret = new CacheResult();

        ret.httpStatusCode = statusCode;