function headers( opts ) {
	var out = {};
	if ( opts.useragent ) {
		debug( 'User-Agent: %s', opts.useragent );
		out[ 'User-Agent' ] = opts.useragent;
	}
	if ( opts.accept ) {
		debug( 'Accept: %s', opts.accept );
		out[ 'Accept' ] = opts.accept;
	}
	if ( opts.token ) {
		out[ 'Authorization' ] = 'token ' + opts.token;
	}
	out[ 'Content-Type' ] = 'application/json';
	return out;
}
Returns request headers based on provided options.

@private
@param {Options} opts - provided options
@param {string} [opts.token] - GitHub access token
@param {string} [opts.accept] - media type
@param {string} [opts.useragent] - user agent string
@returns {Object} request headers