Fixed validation of last date vs item date.
Found a case for RSS compliant generated by Bamboo CI (example), where the last date was undefined.
Meaning at a moment  it was doing an undefined equals undefined validation, returning empty results.
Quick workaround for this issue.
fixed -
self.options.lastDate should not be updated if date is undefined.
Merge pull request #7 from lmorenojr/master

Fixed validation of last date vs item date.