Invalid JSON attributes still work

[lroellin]

We need to check the request against a known schema before proceeding with them.

The following works (unfortunately):

1. Send a Create a new expense request, but omit the "title" attribute.
2. Observe that

• The request succeeds
• When Getting an expense with the received ID, the title is empty
• It is actually empty in the database
• The value of that attribute in the DTO class is actually null.

Suggested fix:
Create a schema we can check against.
or
Create a instance method checking all fields are populated.