Replace deprecated setID with setCode

Fix for failing `InvoiceTest::testInvoiceMapperRespondsWithInvoiceEntity`
Using `setID` makes the tests fails because it is deprecated, with message:

    "setID is a deprecated function: Use setCode".

Using `setCode` seems the better option
Merge pull request #4 from NanneHuiges/repairtest

Replace deprecated setID with setCode