forces: Updated reading of 'nu'

Resolves bug-report http://bugs.openfoam.org/view.php?id=2158

src/functionObjects/forces/forces/forces.C

@@ -291,7 +291,12 @@ Foam::tmp<Foam::volScalarField> Foam::functionObjects::forces::mu() const
         const dictionary& transportProperties =
              obr_.lookupObject<dictionary>("transportProperties");
 
-        dimensionedScalar nu(transportProperties.lookup("nu"));
+        dimensionedScalar nu
+        (
+            "nu",
+            dimViscosity,
+            transportProperties.lookup("nu")
+        );
 
         return rho()*nu;
     }