Fixes to the has_many finder builder

[JKring]

This is my first PR to any Rails project, so let me know if I'm overstepping my bounds or if I missed the memo about how all this works.

I am attempting to resolve two bugs in the has_many association (one documented, one not).

The undocumented bug is that if you call a has_many association on a new record, it returns the collection parser for that record's class. It should probably return the collection parser for the associated class, no?

The other bug is documented here and here. In short, the documentation says:

# If the response body does not contain an attribute matching the association name
# a request sent to the index action under the current resource.
# For the example above, if the comments are not present the requested path would be:
# GET /posts/123/comments.xml

But that does not seem to be the case. In actuality, the behavior generates a URL like:

GET comments.xml?post_id=123

I believe this PR fixes both those bugs. But I will do further testing to be sure.

[rafaelfranca]

Thank you for the pull request. I still unsure what if it is the doc that is wrong or the code. I'll investigate.

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