Use uint32 for kreditsWithdrawn

Same as the properties and functions in the Contribution contract

test/contracts/Contributor.js

@@ -95,10 +95,8 @@ describe("Contributor contract", async function () {
       expect(c['balance']).to.equal(0);
       await Contributor.connect(addr1).withdraw();
       c = await Contributor.getContributorById(2);
-      // TODO Are we required to use Wei for ERC20 tokens?
       expect(c['balance'].toString()).to.equal("6500000000000000000000");
-      // FIXME This is also a BigNumber (uint256), but stored at Wei reolution
-      expect(c['kreditsWithdrawn'].toString()).to.equal('6500');
+      expect(c['kreditsWithdrawn']).to.equal(6500);
     });
 
     it("requires the withdrawable amount to be larger than 0", async function () {