ST_AddMeasure refuses to set measure of zero-length line

[strk]

In modeling the trajectory of a non-moving object it is useful to be able to create a LINESTRING with 2 vertices that have the same XYZ portion but different M. Using ST_AddMeasure on such a topologically invalid input results in the M being set to 0 in both first and second (last) vertex:

=# select ST_AsText(ST_AddMeasure('LINESTRING(0 0,0 0)', 1, 2));
         st_astext          
----------------------------
 LINESTRING M (0 0 0,0 0 0)
(1 row)

=# select ST_AsText(ST_AddMeasure('LINESTRING(0 0,1 0)', 1, 2));
         st_astext          
----------------------------
 LINESTRING M (0 0 1,1 0 2)
(1 row)

I think it would be nice for the first case to return

LINESTRING M (0 0 1,0 0 2)

I know, it's a very special case, but it seems to be well defined.

[pramsey]

Fixed at r13830