Implement #or to either

[moba1]

Abstract

Implement #or method to Either(T) and add test.
If receiver is Right, this method returns value which is same as receiver, otherwise, returns Either monad which is argument.

Example

# If `value` is `Right`, `#or` return `value`
value = Right.new(1)
value.or(Left.new("string")) == Right.new(1)

# If `value` is `Left`, `#or` return argument
value = Left.new(1)
value.or(Right.new("string")) == Right.new("string")