[Manifest] Unclear if "should" intended as "SHOULD"

[RobDolinMS]

size int
...
If the length of the retrieved content does not match the specified length, the content should not be trusted.

If intended as a recommendation, use "SHOULD" (ALL CAPS.)
If not intended as a recommendation invoke RFC meaning, consider alternate wording.

[stevvooe]

I'm not even sure this is correct. The size is really there to support preallocation. Content verification should be done through digest.

[philips]

I think it is correct. This may protect against future length extension attacks.

[stevvooe]

Ok, this should be SHOULD, then.

I'm going to add this to #111, as this question comes up all the time.

[philips]

confirmed fixed in #111