title: Apollo Client: cache.modify does not work?
body: <p>I can't seem to get my cache to update after a mutation, although I passed the typename and id to reference the object to modify. What am I doing wrong?
  I reproduced a simple example here.</p>
  <p>I first initialized my cache with the data -</p>
  <pre><code>const writeInitialData = () =&gt; {
      cache.writeQuery({
          query: gql`
              query {
                  test
              }
          `,
          data: {
              test: {
                  __typename: 'test',
                  id: 'id1',
                  string: 'initial string',
              }
          },
      });
  };
  </code></pre>
  <p>I then executed the mutation on my client.</p>
  <pre><code>updateTest: (_, __, { cache }) =&gt; {
      cache.modify({
        id: cache.identify({
          __typename: 'test',
          id: 'id1',
        }),
        fields: {
          string: 'changed string',
        },
      });
  }
  </code></pre>
  <p>however, the value in the cache did not change, and furthermore if I console.log() the returned value of cache.modify, it gives me <code>false</code>, implying that there is no update to the cache.</p>