StepRange constructor with an OrdinalRange argument

[jishnub]

The StepRange constructor currently doesn't convert from a StepRange, but similar calls work with other range types such as StepRangeLen and UnitRange.

julia> StepRange(2:1:4)
ERROR: MethodError: no method matching StepRange(::StepRange{Int64, Int64})
The type `StepRange` exists, but no method is defined for this combination of argument types when trying to construct it.

After this PR, it works:

julia> StepRange(2:1:4)
2:1:4