Use Time.parse_safely in Feed.last_modified_from_header.
Use Time.parse_safely in Feed.last_modified_from_header.

This helps prevent parse failures when there is a problem with the last
modified header. For example if the Last-Modified header is empty like on
http://feeds.kottke.org/main.

bakshi41c/feedjira,jfiorato/feedjira,davoclavo/feedjira,pulkit21/feedjira,zenati/feedjira,iitgiq/feedjira,IFTTT/feedjira