function headers( opts ) {
	var out = {};
	if ( opts.useragent ) {
		debug( 'User-Agent: %s', opts.useragent );
		out[ 'User-Agent' ] = opts.useragent;
	}
	return out;
}
Returns request headers based on provided options.

@private
@param {Options} opts - provided options
@param {string} [opts.useragent] - user agent string
@returns {Object} request headers