Make Sheet#each_row return Enumerator when no block

[myabc]

This allows for chaining Enumerators, e.g.

sheet.each.with_index do |row, index|
    # ...
end

and matches behaviour of Array#each/Hash#each

enum = [1, 2, 3].each # => #<Enumerator: ...>
enum.peek # => 1

[martijn]

Great idea, thanks for contributing!