Mismatching `sizeof()` for date inner values

[michaelwright235]

I've encountered a bug when I tried using the plist_set_date_val function:

Assertion failed: (length == sizeof(double)), function plist_get_date_val, file plist.c, line 1420.

When I was expecting the code, I found out that arguments of sizeof for date related functions are mismatching.

plist_new_date(...) (uses double):

data->length = sizeof(double);

plist_set_date_val(...) (uses timeval):

plist_set_element_val(node, PLIST_DATE, &val, sizeof(struct timeval));

plist_get_date_val(...) (uses double):

assert(length == sizeof(double));