If a mutator has no return value (i.e., returns undefined), leave t…

…he subject unchanged (useful for in-place updates)

src/create-cycle.js

@@ -14,14 +14,14 @@ export default function createCycle(renderer, data={}) {
  // optionally key-specific mutation of data
  // eg: mutate('value', v => v*2 )
  function mutate(fn, ...args) {
- let key;
+ let key, r;
  if (typeof fn==='string') {
  key = fn;
  fn = args.splice(0, 1)[0];
  }
  let p = key ? data[key] : data;
- if (typeof fn==='function') p = fn(p, ...args);
- else p = fn;
+ if (typeof fn!=='function') p = fn;
+ else if ( (r=fn(p, ...args))!==undefined ) p = r;
  if (key) data[key] = p;
  else data = p;
  if (!debounce) debounce = setTimeout(render, 1);