The method according to claim 4, wherein, in step c), the reducing agent is selected from the group consisting of hydrazine monohydrate (H2NNH2.H2O); compounds containing hydrazine monohydrate (H2NNH2.H2O); and organic alkaline compounds represented by R???NHn wherein R is a C1???20 alkyl or alkoxy group, and n is an integer of from 0 to 3. 9.