Handle single child when using React inlining

Changes optimisation.react.inlineElements to handle a single child as the
value of the `children` property instead of wrapping it with an array.
This matches the behavior of `React.createElement`.

test/core/fixtures/transformation/optimisation.react.inline-elements/nested-html-elements/expected.js

@@ -4,7 +4,7 @@
   type: "div",
   ref: null,
   props: {
-    children: [bar],
+    children: bar,
     className: "foo"
   },
   key: null