handleFetch not always needed

[amacleay]

If a consumer provides handleResponse, they generally don't need to provide handleFetch:

// ERROR: missing property handleFetch
// even though there is no need for it here
new TokenRefreshLink({
  fetchAccessToken: () => {
    const refreshedSession = refreshSession();
    const fakeReq = new Request('uri://fake');
    return Object.assign(fakeReq, refreshedSession);
  },
  handleResponse: (_, __) => response => response.text()
      .then(JSON.parse)
      .then(res => {
        saveToken(res.accessToken);
      }),
  },
  handleError: console.error,
});

Expected behavior:
The constructor should enforce that at least one of handleFetch and handleResponse are present

[newsiberian]

Hi, this code will fails because there is a check between handleResponse and handleFetch functions that expect to find a data we marked as accessTokenField, but I see your point. Originaly handleResponse was internal

I'm not sure the way you suggested will not confuse people, if we will mark handleFetch as an optional.

What do you think?