Add unit tests for POST requests of the REST API.

[UmenR]

Currently, the unit tests that have been written for the REST API [1] consists of tests that cover HTTP GET requests. This means only the handler methods that do information retrieval are covered by these unit tests.

We need to add unit tests that cover handler methods involved in the creation/manipulation of data.

[1]https://github.com/elimu-ai/webapp/tree/master/src/test/java/ai/elimu/rest/v2