[Store] Documents might contain stale values after update

[Merkur39]

Today, documents in the Store are updated in this way:

const merged = {
  ...prevDocument,
  ...nextDocument
}

if (prevDocument.relationships || nextDocument.relationships) {
  merged.relationships = {
    ...prevDocument.relationships,
    ...nextDocument.relationships
  }
}

NB: in an object literal, the spread syntax enumerates the properties of an object and adds the key-value pairs to the object being created. But at the first level*

This approach allows you to update the document in the Store without necessarily having to pass the whole document to it (nextDocument), but poses a problem when you want to delete an attribute at the second level (or at the second level of relationships).

This change was made during this PR: #342

1. We'll have to see why this problem has been corrected here and not elsewhere.
2. We probably shouldn't have a spread, and replace the Store document with the new one. But we need to make sure we always have the full document in nextDocument.