Allow non-positive integer delays for immediate backoff strategy

Per [the
documentation](https://github.com/mixmaxhq/bee-queue#jobbackoffstrategy-delayfactor)
you should be able to call `job.backoff('immediate')` to set an
immediate backoff strategy for retried jobs. However, the `backoff`
method explicitly disallowed this by checking for
`Number.isSafeInteger(undefined)` which returns false. This means
you currently cannot specify an immediate backoff strategy without also specifying a positive delay value.

To fix this, only check for valid integers when the strategy is not
`immediate`.

lib/job.js

@@ -124,7 +124,9 @@ class Job extends Emitter {
     if (!strategies.has(strategy)) {
       throw new Error('unknown strategy');
     }
-    if (!Number.isSafeInteger(delay) || delay <= 0) {
+
+    const isInvalidDelay = !Number.isSafeInteger(delay) || delay <= 0;
+    if (strategy !== 'immediate' && isInvalidDelay) {
       throw new Error('delay must be a positive integer');
     }
     this.options.backoff = {

test/queue-test.js

@@ -1464,6 +1464,40 @@ describe('Queue', (it) => {
       t.true(calls[1] - calls[0] >= 100);
     });
 
+    it('should handle immediate backoff', async (t) => {
+      const queue = t.context.makeQueue({
+        activateDelayedJobs: true
+      });
+
+      const calls = [];
+
+      queue.process(async (job) => {
+        t.deepEqual(job.options.backoff, {
+          strategy: 'immediate'
+        });
+        t.deepEqual(job.data, {is: 'immediate'});
+        calls.push(Date.now());
+        if (calls.length === 1) {
+          throw new Error('forced retry');
+        }
+        t.is(calls.length, 2);
+      });
+
+      const succeed = helpers.waitOn(queue, 'succeeded', true);
+
+      await queue.createJob({is: 'immediate'})
+        .retries(2)
+        .backoff('immediate')
+        .save();
+
+      await succeed;
+
+      t.is(calls.length, 2);
+
+      // Ensure there no delay.
+      t.true(calls[1] - calls[0] <= 100);
+    });
+
     it('should handle exponential backoff', async (t) => {
       const queue = t.context.makeQueue({
         activateDelayedJobs: true