BUG: fix genfromtxt check of converters when using usecols

fixes an issue reported by Adrian Altenhoff where user-supplied
converters in genfromtxt were not tested with the right first_values
when also specifying usecols.
Merge pull request #5006 from dhomeier/ioconv_usecols

BUG: fix genfromtxt check of converters when using usecols