Caching for Blob object

[kygoh]

Added a method to cache request that returns blob object.

Use case:

	const image = document.createElement('img');

	const request = this.httpClient.get(imageURL, {responseType: 'blob'});

	this.cache.loadFromBlobObservable(imageURL, request)
	.subscribe(response => image.src =  URL.createObjectURL(response));

[Nodonisko]

Thanks, looks great!