2.x concatMapEager should accept 0 for prefetch

[ansman]

The documentation for concatMapEager states:

@param prefetch hints about the number of expected source sequence values

Which leads me to assume that the following code should work:

Observable.fromIterable(someCollection)
    .concatMapEager(this::performTask, 5, someCollection.size())

but it will fail if the collection is empty. Either the documentation should be updated to reflect that 0 is not a valid value or it should accept 0 (and perhaps set it to a default value internally).

[akarnokd]

Looks like there is a missing validation there as well. PR welcome. Plus, the parameter document should read something like:

hints about the number of expected values from each inner ObservableSource

[akarnokd]

On a second look, this is not a bug in the operator but perhaps the underdocumentation. Prefetch of 0 or negative has no meaning and if you think there won't be any value, use 1 as the prefetch amount.

[akarnokd]

Closing via #5189.