_setHeader = ( config, header, value ) => ( {
	...config,
	headers: {
		...( config && config.headers ? config.headers : null ),
		[ header ]: value,
	},
} )
Utility method to set a header value on a fetch configuration object.

@method _setHeader
@private
@param {Object} config A configuration object of unknown completeness
@param {string} header String name of the header to set
@param {string} value  Value of the header to set
@returns {Object} The modified configuration object