Fix Record iteration when recording only zeros

Fixes #124

As pointed in the comment in the corresponding issues, we want to ensure
we trigger the picker logic at least once to keep the invariant of the
loop, that is to say that the picking logic has been executed for
last_picked_index, which is not the case for zero when there are only
zeros.

I added a test that collect the record and check for their count.

Would it be better to check for the following:

```rust
assert_eq!(h.iter_recorded().collect::<Vec<_>>(), vec![IterationValue::<u64>::new(0, 1.0, 1.0, 1, 1)]);
```

It is easy to assume that `h.iter_recorded().collect::<Vec<_>>().len() == h.len()`
and simplify the added test, and I'm unsure how to convey that we
_really_ want to collect and check the length of all the record.

src/iterators/mod.rs

@@ -186,7 +186,7 @@ where
             }
 
             // Have we already picked the index with the last non-zero count in the histogram?
-            if self.last_picked_index >= self.max_value_index {
+            if self.last_picked_index >= self.max_value_index && self.last_picked_index != 0 {
                 // is the picker done?
                 if !self.picker.more(self.current_index) {
                     self.ended = true;

tests/histogram.rs

@@ -560,3 +560,10 @@ fn subtract_underflow_guarded_by_per_value_count_check() {
         h.subtract(h2).unwrap_err()
     );
 }
+
+#[test]
+fn recorded_only_zeros() {
+    let mut h = Histogram::<u64>::new(1).unwrap();
+    h += 0;
+    assert_eq!(h.iter_recorded().collect::<Vec<_>>().len(), 1);
+}