Fix: Add support for rendering multiple scopes by passing string as names

[swilgosz]

Background

Resolves: #252

Bug 1.

When scope name is passed as string (either scope class name, or the path to the file), the name resolving goes through the cache, with a hardcoded key scope_class, not allowing mutliple scopes to be stored and fetched.

Bug 2.

If namespace is not defined (nil), const_defined? is called on nil, causing Undefined method error to be risen.

Design

• Add spec covering both issues detected.
• Change the hardcoded cache key to the one taking passed name into the consideration
• Add support to nil namespace, falling back to the Object, so we can use const_get and const_defined? methods.