zcap has incomplete expiration checks in capability chains.

`@digitalbazaar/zcap` provides JavaScript reference implementation for Authorization Capabilities. Prior to version 9.0.1, when invoking a capability with a chain depth of 2, i.e., it is delegated directly from the root capability, the `expires` property is not properly checked against the current date or other `date` param. This can allow invocations outside of the original intended time period. A zcap still cannot be invoked without being able to use the associated private key material. `@digitalbazaar/zcap` v9.0.1 fixes expiration checking. As a workaround, one may revoke a zcap at any time.