Fix shared route bindings mutation in _substituteBindings

[pataar]

When _substituteBindings encounters an object missing its registered binding key (e.g. slug), it falls back to id — but it does so by writing bindings[key] = 'id' directly on the shared route definition. This permanently overwrites the original binding for all subsequent calls.

For example, with a posts.show route bound to slug:

// First call — partial object, no slug, falls back to id
route('posts.show', { post: { id: 1 } });
// 'https://example.com/posts/1' ✓

// Second call — full object with slug
route('posts.show', { post: { id: 1, slug: 'hello-world' } });
// Expected: 'https://example.com/posts/hello-world'
// Actual:   'https://example.com/posts/1' ✗

The second call uses id instead of slug because the first call mutated the route config.

This PR replaces the mutation with a local variable so the fallback is resolved per-call.

[bakerkretzmar]

Wow great catch, thank you!