Prevent dplyr from joining on NA's : <p>I'd like to do a full-join of 2 df's.  To my surprise, dplyr's default behavior is to join on NA's if they exist in both df's.  Is there a functionality to prevent dplyr from doing this?</p>

<p>Here's an example with inner join:</p>

<pre><code>x &lt;- data.frame(a = c(5, NA, 9), b = 1:3)
y &lt;- data.frame(a = c(5, NA, 9), c = 4:6)
z &lt;- dplyr::inner_join(x, y, by = 'a')
</code></pre>

<p>I would like z to contain only 2 records, not 3.  Ideally, I want to do this without having to manually filter out the records with NA's beforehand and then append them to the result (since that seems clumsy).</p>
