MutationVisitor doesn't follow assignment value

Looks like the `MutationVisitor#visit_assign ` doesn't follow the node's `value`.

```ruby
def visit_assign(node)
  node.copy(target: visit(node.target))
end
```

Is this intentional? Here's a fix.

```ruby
def visit_assign(node)
  node.copy(target: visit(node.target), value: visit(node.value))
end
```

I can make a PR if you need.