Incorrect endpoints before the epoch

[joshuaulrich]

endpoints returns incorrect values when it is called on an xts object with index values prior to the Unix epoch. For example:

require(xts)
x <- .xts(1:22, 1.0*(-10:11), tzone="UTC")
merge(x, ep=x[endpoints(x, 'seconds', 2)])
                     x ep
1969-12-31 23:59:50  1  1
1969-12-31 23:59:51  2 NA
1969-12-31 23:59:52  3  3
1969-12-31 23:59:53  4 NA
1969-12-31 23:59:54  5  5
1969-12-31 23:59:55  6 NA
1969-12-31 23:59:56  7  7
1969-12-31 23:59:57  8 NA
1969-12-31 23:59:58  9  9
1969-12-31 23:59:59 10 NA
1970-01-01 00:00:00 11 NA
1970-01-01 00:00:01 12 12
1970-01-01 00:00:02 13 NA
1970-01-01 00:00:03 14 14
1970-01-01 00:00:04 15 NA
1970-01-01 00:00:05 16 16
1970-01-01 00:00:06 17 NA
1970-01-01 00:00:07 18 18
1970-01-01 00:00:08 19 NA
1970-01-01 00:00:09 20 20
1970-01-01 00:00:10 21 NA
1970-01-01 00:00:11 22 22

I think the cause of the issue is that casting from double to long truncates toward zero. This is desirable for positive numbers (moving the endpoint back in time). But when the index value is negative, truncating toward zero moves the endpoint forward in time.

This is also the case for integer division in the C99 standard, so the fix should be applied for both integer and double index types.