ST_LineMerge with collection

[strk]

According to GEOS dox about LineMerger:

• Any dimension of Geometry is handled.
• The constituent linework is extracted to form the edges.

According to PostGIS manual:

Only use with MULTILINESTRING/LINESTRINGs. If you feed a polygon or geometry collection into this function, it will return an empty GEOMETRYCOLLECTION

Reality:

SELECT ST_AsText(ST_LineMerge(
'GEOMETRYCOLLECTION(
   MULTILINESTRING(
    (1656318.45 4833344.45,1656321.79 4833339.62,1656312.54 4833333.49),
    (1656312.54 4833333.49,1656309.68 4833337.07)
   ),
   LINESTRING(1656309.68 4833337.07,1656318.45 4833344.45)
)'));
                        st_astext                        
---------------------------------------------------------
 LINESTRING(1656309.68 4833337.07,1656318.45 4833344.45)
(1 row)

Basically, the MULTILINESTRING part was simply _discarded_. Works fine if the input is transformed in a MULTILINESTRING (without the collection).

I'm not sure if the bug is in postgis or geos, will check. Surely dox of postgis and geos disagree on the matter.