Fix RBS translation of shape string keys

[Morriar]

Before this change all shape keys were always translated as symbol, this change makes it possible to use string keys in shapes.

So this RBI signature:

sig { returns({a: A, "b": B, :c => C, "d" => D}) }
def foo; end

Is properly translated as:

def foo: -> {a: A, b: B, c: C, "d" => D}