Fixes choojs/nanohtml#51. 'false' no longer renders

[mjstahl]

This is a breaking change.

Alright, following the calls of concat I found where to make the change (I believe). Thank you @goto-bus-stop

I did find an error in the process and added a test and fix for it (reference https://github.com/choojs/hyperx/pull/79/files#diff-9b0f7b44960ccf14de3ba5c706daf2b4R30). Results in:

/Users/mark/Projects/JavaScript/hyperx/node_modules/virtual-dom/virtual-hyperscript/index.js:77
        throw UnexpectedVirtualElement({
        ^

Error: Unexpected virtual child passed to h().

This is because of what virtual-dom supports (https://github.com/Matt-Esch/virtual-dom/blob/master/virtual-hyperscript/index.js#L74).