Bug report: collection level headers take precedence

[jonrosner]

When setting up collection-level headers, the behavior should be that request-level headers overwrite collection-level headers with the same key. This is currently not the case.

Steps to reproduce:

1. Create a collection
2. Set a collection-level header
3. Create a request
4. Set a request-level header with the same key
5. Send the request, and observe that the collection-level header was used