Add unit test.
Add unit test.

New test is currently disabled for complex variables, but it should
not be too hard to add support for that by looking at how it's done
for write_element_data().

BalticPinguin/libmesh,jwpeterson/libmesh,capitalaslash/libmesh,dschwen/libmesh,capitalaslash/libmesh,hrittich/libmesh,hrittich/libmesh,libMesh/libmesh,90jrong/libmesh,BalticPinguin/libmesh,hrittich/libmesh,roystgnr/libmesh,libMesh/libmesh,hrittich/libmesh,pbauman/libmesh,pbauman/libmesh,dschwen/libmesh,jwpeterson/libmesh,dschwen/libmesh,hrittich/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,pbauman/libmesh,dschwen/libmesh,90jrong/libmesh,capitalaslash/libmesh,90jrong/libmesh,libMesh/libmesh,jwpeterson/libmesh,90jrong/libmesh,hrittich/libmesh,dschwen/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,dschwen/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,hrittich/libmesh,libMesh/libmesh,libMesh/libmesh,capitalaslash/libmesh,pbauman/libmesh,jwpeterson/libmesh,pbauman/libmesh,capitalaslash/libmesh,capitalaslash/libmesh,dschwen/libmesh,90jrong/libmesh,pbauman/libmesh,90jrong/libmesh,hrittich/libmesh,roystgnr/libmesh,90jrong/libmesh,libMesh/libmesh,capitalaslash/libmesh,pbauman/libmesh,dschwen/libmesh,roystgnr/libmesh,jwpeterson/libmesh,pbauman/libmesh,roystgnr/libmesh,jwpeterson/libmesh,90jrong/libmesh,libMesh/libmesh,90jrong/libmesh,BalticPinguin/libmesh,hrittich/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,roystgnr/libmesh,roystgnr/libmesh,libMesh/libmesh,pbauman/libmesh,roystgnr/libmesh,jwpeterson/libmesh