Fix for bug 1258, manual tax on invoice screen did not respect number formats

--HG--
extra : source : e5310245cde61ea4645b3643dc180a2d626399d9
Amending changelog for 1.3-only fix

--HG--
extra : source : 234388d09430302c3d9ae11ec51caa272262d4b3
Merge pull request #476 from einhverfr/master

including fix for 1258