Improve .retry() method functionality with delays and prevent calling on successful request

[IAMtheIAM]

This PR adds new functionality:

• The ability to specify the callback of .retry() as an async function. The purpose is to enable awaiting a Promise within the callback. This could be to used to fetch some external data, or by simply utilizing a setTimeout to resolve the promise after a specified delay. The result is that the .retry() is delayed by the given amount (or after the data is received). It does not break existing functionality i.e. the .retry() method can still be a normal synchronous function without any difference in behavior.

This PR also Resolves #1526

• It prevents the .retry() method from being invoked on a successful request, and only allows it to invoke when there is a specified non-null error passed to .callback()

[IAMtheIAM]

I'm not understanding why the build is failing, can someone explain?
The tests are not setup for us to run locally. There is no localhost:5000 so I cannot run any tests.

I've spent half a day trying to get the tests to pass but cannot. So I need some help. The code does work as far as I've manually tested.

[niftylettuce]

@IAMtheIAM it seems that the tests fail due to unhandled promise rejection related to retry tests https://travis-ci.org/visionmedia/superagent/jobs/610131622?utm_medium=notification&utm_source=github_status

if you fix + also fix merge conflicts (perhaps rebase to latest in our master branch and then add your work on top is best approach) - I would def merge and release this fix

[niftylettuce]

Closing until you submit a better PR

[niftylettuce]

Re-opened

[niftylettuce]

ping @3imed-jaberi

[IAMtheIAM]

@niftylettuce I resolved the merge conflicts by rebasing master onto my branch
I'm not sure about resolving the test yet, I'll take a look.

Update: The build failed with some lock file issue. Do you know what it means?

[niftylettuce]

Yes you forgot to update yarn lockfile and also you were missing adding @babel/plugin-transform-runtime to dev dependencies

[niftylettuce]

One moment, I'm fixing

[niftylettuce]

@IAMtheIAM can you please pull the latest from master branch? I pushed commit 0c4f96f but the tests are still failing. Please resolve and let me know.

[niftylettuce]

@IAMtheIAM see https://travis-ci.org/github/visionmedia/superagent/jobs/709314173 for failing tests

[niftylettuce]

I have reverted 62eae78 until tests are passing and this PR is re-submitted.

[niftylettuce]

I'm going to leave the revert as is. This would be a major breaking change, as some may be using the retry callback to retry the request completely, even if no error occurred. Albeit it should be intended that a retry only happens when an error occurs, however we should leave this up to the user to override. If you do not want anything that has an error to even hit the shouldRetry function, then you could make a custom retry function callback.

[niftylettuce]

With regards to the async/promise support, that would be nice to have, but it seems your PR broke all the tests. If you can fix this and then re-submit (taking out the err !== null portion, then let me know.