Added the posibility that the user creates his own date parser when reading a feed...

Added the posibility that the user creates his own date parser when reading a feed from outside, also as part of our default date parser, if it can't parse the date it will just assign a default date to avoid the crash

Commit migrated from https://github.com/dotnet/corefx/commit/37bf505927c3aad98e24872bc12dc3cebd6b5510