RequestImpl.ifModifiedSince not comparing same precisions

The implementation of RequestImpl.ifModifiedSince takes a date as a String and another as a Date. The String date has precision up to seconds (because its an HTTP header), but the Date argument may have milliseconds.

Thus, comparing these dates using getTime() (millisecond precision) will always (almost) be false. The comparison should use precision up to the second since this is the most HTTP headers can provide.