expose a method to set updated date of feed

[guanpu]

Sometimes it's necessary that the "updated" field of the feed needs to be updated, like adding New Item or so. So I exposed a new method so that the "updated" field is possible to be updated.

[coveralls]

Coverage increased (+0.004%) to 99.574% when pulling aeb048c5b4873b82830904a7d25192cdb193c789 on guanpu:master into fd77835d23990670975092c15009c75432e258ac on jpmonette:master.