Fix include parser dropping sibling sub-includes under a shared intermediate relationship

[albertoforcato]

When two or more include paths share the same intermediate relationship node (e.g. bestComments.user.company and bestComments.user.topPosts), the parser processes each path separately and accumulates results one by one with Keyword.merge/2. Because both paths produce the same top-level key (best_comments: [user: [...]]), the second merge silently overwrites the first, dropping earlier sub-preloads from the Ecto preload list.

The fix replaces the shallow Keyword.merge/2 with a recursive deep_merge/2 that combines nested keyword lists instead of overwriting them, so every leaf preload is retained regardless of ordering or nesting depth.

Added four tests to parse_include/2 covering:

• two siblings under the same parent
• siblings when the intermediate node is also requested explicitly
• four paths sharing intermediate nodes at multiple depths

[lucacorti]

This is great, thank you!