BUG: fix ma.minimum.reduce with axis keyword
BUG: fix ma.minimum.reduce with axis keyword

Fixes the problem reported at
https://github.com/numpy/numpy/pull/21977#issuecomment-1186082534

The reduce method here effectively calls itself with an unmasked
MaskedArray (mask=nomask) and then expects either a MaskedArray or
a scalar.  This change ensures that an ordinary ndarray is
converted to a MaskedArray, following the pattern already used in
mean and var in this module.

mattip/numpy,endolith/numpy,mattip/numpy,mattip/numpy,anntzer/numpy,charris/numpy,mhvk/numpy,mhvk/numpy,anntzer/numpy,anntzer/numpy,numpy/numpy,charris/numpy,endolith/numpy,endolith/numpy,mattip/numpy,numpy/numpy,numpy/numpy,mhvk/numpy,anntzer/numpy,charris/numpy,endolith/numpy,charris/numpy,mhvk/numpy,numpy/numpy,mhvk/numpy