mapwindow failure cases

[yha]

Some problems I came across while testing for JuliaImages/ImageFiltering.jl#160:

• border "reflect" seems to fail for non-symmetric windows

  mapwindow(minimum, 1:8, -2:0; border="reflect") # DimensionMismatch

• borders NA() and NoPad() don't work

  mapwindow(minimum, 1:8, 3; border=NA()) # MethodError
  mapwindow(minimum, 1:8, 3; border=NoPad()) # MethodError

• fails when given indices entirely inside the border:

  mapwindow(minimum, 1:8, -2:0; indices=1:2) # AssertionError on master, DimensionMismatch with JuliaImages/ImageFiltering.jl#160