Implement web server unit tests

[binerys]

• Write unit tests for your webserver.
• All of your web server code should have unit tests.
• Every class should have a unit test.
• Every .cc file should have a _test.cc file.
• Exception: your main() method doesn't need unit tests, but it should be trivial.
• Use dependency injection test code in isolation. (Refer to the example from Lecture 2)
• If you find your code is difficult to test, refactor it to improve testability.
• Your Makefile should make it easy to run all your tests. For example, make test.