FIX - Output of flip on irregular time axis is inconsistent

[Bang Pham Huu]

For test WCPS 503-flip_on_3d_time_axis.test

for $c in (test_wms_3d_time_series_irregular)
return encode(flip $c[Lat:"CRS:1"(0:0), Long:"CRS:1"(0:0)] + 20 ALONG unix, "application/gml+xml")

it returns first run correctly with:

<gmlrgrid:coefficients>"2015-07-01T00:00:00.000Z" "2015-05-01T00:00:00.000Z" "2015-04-01T00:00:00.000Z" "2015-03-01T00:00:00.000Z" "2015-02-01T00:00:00.000Z" "2015-01-01T00:00:00.000Z"</gmlrgrid:coefficients>

then wrongly in the second run with:

<gmlrgrid:coefficients>"2015-07-01T00:00:00.000Z" "2015-08-01T00:00:00.000Z" "2015-08-29T00:00:00.000Z" "2015-09-29T00:00:00.000Z" "2015-10-29T00:00:00.000Z" "2015-12-29T00:00:00.000Z"</gmlrgrid:coefficients>