Strip zero padding from ETag strings

I have no idea why I haven't done this in the first place: Zero padding in Base64 is serving no purpose other than padding the resulting string length to an integer multiple of four. The informational value of those trailing `=`-signs is nil. Hence they can be safely stripped, effectively shaving off a few bytes on each cacheable response.

That being said, due to the changed strings, there is going to be a penalty of one uncached request per resource for caching clients, which I find quite acceptable.