Copy assignment operator should return *this.
Copy assignment operator should return *this.

Fixes #1338.

svallaghe/libmesh,dschwen/libmesh,giorgiobornia/libmesh,hrittich/libmesh,dschwen/libmesh,pbauman/libmesh,jwpeterson/libmesh,capitalaslash/libmesh,balborian/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,90jrong/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,90jrong/libmesh,giorgiobornia/libmesh,vikramvgarg/libmesh,giorgiobornia/libmesh,dschwen/libmesh,giorgiobornia/libmesh,hrittich/libmesh,svallaghe/libmesh,capitalaslash/libmesh,pbauman/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,balborian/libmesh,BalticPinguin/libmesh,giorgiobornia/libmesh,balborian/libmesh,hrittich/libmesh,jwpeterson/libmesh,libMesh/libmesh,pbauman/libmesh,svallaghe/libmesh,roystgnr/libmesh,hrittich/libmesh,libMesh/libmesh,jwpeterson/libmesh,pbauman/libmesh,balborian/libmesh,dschwen/libmesh,roystgnr/libmesh,90jrong/libmesh,BalticPinguin/libmesh,libMesh/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,vikramvgarg/libmesh,hrittich/libmesh,roystgnr/libmesh,dschwen/libmesh,90jrong/libmesh,vikramvgarg/libmesh,90jrong/libmesh,pbauman/libmesh,vikramvgarg/libmesh,balborian/libmesh,jwpeterson/libmesh,roystgnr/libmesh,vikramvgarg/libmesh,BalticPinguin/libmesh,hrittich/libmesh,90jrong/libmesh,balborian/libmesh,balborian/libmesh,balborian/libmesh,vikramvgarg/libmesh,dschwen/libmesh,libMesh/libmesh,roystgnr/libmesh,svallaghe/libmesh,jwpeterson/libmesh,vikramvgarg/libmesh,hrittich/libmesh,hrittich/libmesh,BalticPinguin/libmesh,svallaghe/libmesh,dschwen/libmesh,vikramvgarg/libmesh,capitalaslash/libmesh,giorgiobornia/libmesh,hrittich/libmesh,roystgnr/libmesh,90jrong/libmesh,giorgiobornia/libmesh,libMesh/libmesh,jwpeterson/libmesh,pbauman/libmesh,libMesh/libmesh,dschwen/libmesh,pbauman/libmesh,svallaghe/libmesh,svallaghe/libmesh,capitalaslash/libmesh,vikramvgarg/libmesh,svallaghe/libmesh,90jrong/libmesh,capitalaslash/libmesh,90jrong/libmesh,pbauman/libmesh,roystgnr/libmesh,pbauman/libmesh,roystgnr/libmesh,balborian/libmesh,svallaghe/libmesh,giorgiobornia/libmesh,balborian/libmesh,giorgiobornia/libmesh,libMesh/libmesh,libMesh/libmesh