core: Use g_ascii_strto[u]ll() instead of strto[u]l()

The GLib functions are locale-independent. While there are few
situations where a locale-independent server-provided string would be
parsed differently by strtoul() in different user locales, it’s best to
be safe (and consistent with our use of g_ascii_strtod(), which more
easily causes problems).
tests: Fix GDataAuthorizer:proxy-resolver property tests

The ClientLogin and OAuth 1 authoriser tests were assuming that the
proxy-resolver property is initially NULL. Actually, the property is
only potentially initially NULL, as it comes from a SoupSession which
the tests have no control over.