AbstractSitemapGenerator.toFile requires directory tree

[cdalexndr]

Currently, using AbstractSitemapGenerator.toFile(file) with a file path where parent directory does not exists results in exception, so the user must manually create parent directory with file.getParentFile().mkdirs() before calling this method.

This check should be embedded inside the method to be user-friendly. For example, Apache IO commons uses:

    public static FileOutputStream openOutputStream(final File file, final boolean append) throws IOException {
        if (file.exists()) {
            if (file.isDirectory()) {
                throw new IOException("File '" + file + "' exists but is a directory");
            }
            if (file.canWrite() == false) {
                throw new IOException("File '" + file + "' cannot be written to");
            }
        } else {
            final File parent = file.getParentFile();
            if (parent != null) {
                if (!parent.mkdirs() && !parent.isDirectory()) {
                    throw new IOException("Directory '" + parent + "' could not be created");
                }
            }
        }
        return new FileOutputStream(file, append);
    }