BUG: solverInfo: ensure access to initial residual fields (#1452)

Prior to the commit, initial residual fields were registered by
the `setResidualField()` function of a linear solver with a field name
prefixed by `residual:`. However, `solverInfo` FO could only access to
the initial residual fields prefixed by `initialResidual:`.

Due to this discrepancy, using `solverInfo` FO with `writeResidualFields=true`
option was resulting in empty residual fields to be output.