SoupDate: Move from GTimeVal to glibc APIs in soup_date_to_time_t()

SoupDate: Move from GTimeVal to glibc APIs in soup_date_to_time_t()

GTimeVal is deprecated, so let's stop using it. Also add a test
for soup_date_to_time_t() which was missing.

The fixes in this change use time_t-specific APIs from libc only.
As such, they are not necessarily Y2038-safe depending on the
architecture. time_t is not safe anyway so there's no point in
trying to be. This method will be deprecated in the next release
cycle anyway.

https://gitlab.gnome.org/GNOME/libsoup/commit/eab0987dc7b46eb4e4c0cf8fe5afcbf6aac18830

Change-Id: I8cb9f2e6f203fe504da4f2079f79cc6b51464046