Add test for ConsulResponse.toString

[lacinoire]

Hey 😊 I want to contribute the following test:

Test that consulResponse.toString() is equal to "ConsulResponse{response=success, lastContact=0, knownLeader=false, index=0, cache=Optional.empty}". This tests the method ConsulResponse.toString. This test is based on the test consulResponseShouldHaveResponseAndDefaultValuesIfNoHeader.

Curious to hear what you think!

Also, is the description I provided of the test helping you to answer to this pull request? Why (not)?

(I wrote this test as part of a research study at TU Delft. Find out more)