constructor (cache, requestData = {}, cacheWhitelist) {
		this.cache = cache;
		this.cacheWhitelist = cacheWhitelist;
		this.requesters = 0;
		this.dfd = Q.defer();
		this.loaded = false;
		this.res = undefined;
		this.err = undefined;
		// copy the rest of the properties from input requestData
		// to this.requestData

		this.url = requestData.urlPath;
		this.requestData = {};
		Object.keys(requestData)
			.forEach(key => {this.requestData[key] = requestData[key]});
	}
@param cache [required] the RequestDataCache instance that owns this
       `CacheEntry`.
@param requestData a request data descriptor. Won't be passed if this
       CacheEntry is in the process of being rehydrated
@param cacheWhiteList [optional] the whitelist of repsonse fields that
       will be serialized with this entry. Not passed when rehydrating.