Unit testing, code coverage, and undefined functions

> wondering if this is an instance where it would be appropriate to let it be less than 100%.

I think it would be appropriate.
Because we can't test a function which does not exists, and probably you don't have to
test `getallheaders()`.

But if you want to get 100% coverage for the class, you can get it:
1. move `_get_request_headers()` method to another class
2. replace the object which calls `getallheaders()` with an mock object only when you run test.

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