An Easy Entry to Dimers [{RuX(μ-X)(CO)(P⌒P)}2] (X = Cl, Br; P⌒P = 1,1‘-Bis(diphenylphosphino)ferrocene, 1,1‘-Bis(diisopropylphosphino)ferrocene) from η3-Allylruthenium(II) Derivatives [RuX(η3-2-C3H4R)(CO)(P⌒P)] (R = H, Me): Efficient Catalyst Precursors in Transfer Hydrogenation of Ketones§