Prevent Maybe constructor from altering the value when Nil (fixes #183)
Prevent Maybe constructor from altering the value when Nil (fixes #183)

As discussed in #183, `maybe(MyType)(x)` currently returns `null` if `Nil.is(x)` returns true.
Notably, this may surprise users that would expect `maybe(MyType)(undefined)` to return `undefined`.
This commit changes the behavior of the `Maybe` constructor, so that `Nil` values are returned untouched by the constructor.
gcanti/tcomb,gcanti/tcomb