H2O-N2 fluid system: also normalize the density if the complex relations are used

this is required to make sure that the relation

\rho^\kappa_g = X^\kappa_g \rho_g

holds unconditionally. note, that we define the sum of the mass
fractions to be the same as the sum of the mole fractions.

(also note, that this change only affects the case where the gas phase
is not present.)

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7271 2fb0f335-1f38-0410-981e-8018bf24f1b0