Copy contract data into new object to avoid object is not extensible error

It seems the new ethers.js version returns contract data as a non-
extensible object. We have to clone it before adding the IPFS data.

lib/contracts/contributor.js

@@ -9,7 +9,8 @@ class Contributor extends Record {
 
   getById (id) {
     return this.functions.getContributorById(id)
-      .then(data => {
+      .then(contractData => {
+        let data = {...contractData};
         data.balanceInt = formatKredits(data.balance);
         return this.ipfs.catAndMerge(data, ContributorSerializer.deserialize);
       });