Fix ElseIfStatement double-evaluation in awaited context

[lahma]

This code in awaited context check caused elseIf.WriteToAsync to always start writing, even when it should be only started if condition is true:

if (!elseIfConditionTask.IsCompletedSuccessfully)
{
    var writeTask = elseIf.WriteToAsync(writer, encoder, context);
    return AwaitedElseBranch(elseIfConditionTask, writeTask, writer, encoder, context, i + 1);

Parametrized test case to test async and non-async paths the get some red tests and then fixed. Now passing the elseIf and write task is retrieved, if needed.

We found out this when running on Linux against sync code, which was quite curious, on Windows it always worked.

[lahma]

@sebastienros this bug was somewhat nasty and unpredictable (template writing both elsif and else content), should there be a new release?