Simplify and optimize the most common methods of 'Headers'

[princerb]

This PR enhances the performance of the modified functions by significantly increasing their execution speed x100000. I used generators of course and I can add more of these kinds in the future if you like!
The current test (tests/models/test_headers.py) can handle this change so I didn't write any.

Before:

$ python -m timeit -r 10 -n 10 -c "from main import h" "h.items()"
10 loops, best of 10: 297 msec per loop

After:

$ python -m timeit -r 10 -n 10 -c "from main import h" "h.items()"
10 loops, best of 10: 660 nsec per loop
:0: UserWarning: The test results are likely unreliable. The worst time (4.01 msec) was more than four times slower than the best time (660 nsec).

If these kind of PRs can be merged, I would like to contribute with like these optimised solutions again to speed up the lib. Or feel free to give a feedback)

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[princerb]

So the tests, I guess then although generators are faster, they are not for good presentation(