detect observables properly when children are passed as part of props…

[rekotiira]

… argument to createElement

Right now karet assumes that children are always passed in React.createElement(type, props, [...children])'s argument children, however it is also possible that children are passed in the props object. For example if you take props as an argument for a component and then spread that as a whole to another component:

const Test = (props) => <div {...props} />

const x = <Test>Hello {Kefir.constant('world')}</Test>

This will yield, when transpiled, in:

var Test = function Test(props) {
  return React.createElement('div', props);
};

var x = React.createElement(
  Test,
  null,
  'Hello ',
  Kefir.constant('world')
);

If children is just a single element, karet will actually work properly, but in case children is an array, like in this example, karet will fail to detect the observable in the array.

This PR addresses the issue.

[codecov-io]

Codecov Report

Merging #4 into master will increase coverage by 0.23%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
+ Coverage   91.24%   91.47%   +0.23%     
==========================================
  Files           1        1              
  Lines         217      223       +6     
==========================================
+ Hits          198      204       +6     
  Misses         19       19

Impacted Files	Coverage Δ
src/karet.js	91.47% <100%> (+0.23%)	✅

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[rekotiira]

Added a test

[polytypic]

LGTM. Thanks!