Replacing "is" with "==" in test_headers.py

[achintya-7]

Python will throw a syntax warning when a comparison is made between String using 'is' keyword. It is better to change the 'is' keyword with '==' for comparisons of String.

https://github.com/ceph/s3-tests/blob/master/s3tests/functional/test_headers.py
In the given file, there are three instances of this problem. There is a case of having a similarity of code in all the files of an Open Source project but the '==' keyword is used in a lot of languages and is a norm.

I can make a PR correcting this simple issue if a maintainer approves of it.