function _setHeaders( config, options ) {
	// If there's no headers, do nothing
	if ( ! options.headers ) {
		return config;
	}

	return objectReduce(
		options.headers,
		( config, value, key ) => _setHeader( config, key, value ),
		config,
	);
}
Set any provided headers on the outgoing request object. Runs after _auth.

@method _setHeaders
@private
@param {Object} config A fetch request configuration object
@param {Object} options A WPRequest _options object
@param {Object} A fetch config object, with any available headers set