Allow pre-1970 dates in If-[Un]Modified-Since

If I want to fetch an object only if it is newer than the first moon
landing, I send a GET request with header:

    If-Modified-Since: Sun, 20 Jul 1969 20:18:00 UTC

Since that date is older than Swift, I expect a 2xx response. However,
I get a 412, which isn't even a valid thing to do for
If-Modified-Since; it should either be 2xx or 304. This is because of
two problems:

(a) Swift treats pre-1970 dates as invalid, and

(b) Swift returns 412 when a date is invalid instead of ignoring it.

This commit makes it so any time between datetime.datetime.min and
datetime.datetime.max is an acceptable value for If-Modified-Since and
If-Unmodified-Since. Dates outside that date range are treated as
invalid headers and thus are ignored, as RFC 2616 section 14.28
requires ("If the specified date is invalid, the header is ignored").

This only works for dates that the Python standard library can parse,
which on my machine is 01 Jan 1 to 31 Dec 9999. Eliminating those
restrictions would require implementing our own date parsing and
comparison, and that's almost certainly not worth it.

Change-Id: I4cb4903c4e5e3b6b3c9506c2cabbfbda62e82f35
Merge "Allow pre-1970 dates in If-[Un]Modified-Since"