ST_LineMerge drops vertical segments

[bfreed3]

SELECT ST_AsText(ST_LineMerge(ST_GeomFromText('MULTILINESTRING((0 0 0, 10 10 10, 20 20 10),(20 20 10, 30 30 20, 30 30 40))')));

The resulting Linestring has a Z dimension, but loses the vertical segments.

This becomes a problem when tracking pipeline construction at places where vertical pipes branch off, for example.

[pramsey]

This is an issue in GEOS, unfortunately, not PostGIS, so I've set the milestone appropriate. To increase odds of a fix rolling in, file a ticket at the GEOS trac too. Thanks!