Make sure NA attributes are rendered as blank. Fixes #99

[wch]

This fixes #99.

Instead of this:

div(class = "abc", class = "xyz", foo=NA)
#> <div class="abc xyz" foo="NA"></div>

It now does this:

div(class = "abc", class = "xyz", foo=NA)
#> <div class="abc xyz" foo></div>

[jjallaire]

I actually have a scenario right now where I depend on the NA behavior to add the async attribute to a <script> tag, e.g. <script async src = "...">. How can I do this after we merge this PR? How do we know that others aren't relying on this behavior?