FIXED: inconsistency of the return value in Context._update

[xtr0]

Fix "_update..then is not a function" error if update is called with unknown url when there is no 404 handler configured

[coveralls]

Coverage remained the same at 93.987% when pulling b5062b8 on xtr0:fix_ctx_update into d8a5fe0 on Profiscience:master.

[coveralls]

Coverage remained the same at 93.987% when pulling 9f07792 on xtr0:fix_ctx_update into d8a5fe0 on Profiscience:master.

[caseyWebb]

I think we still need a way to be able to differentiate between 404s and a beforeNavigateCallback that returns false.