Avoid non-function chai matchers

[jurko-gospodnetic]

I encountered an issue recently with sinon-chai caused by the design choice to make some of its chai matchers implemented as non--function properties.

If you perform an expectation like:

expect(spy).to.have.been.calledOnce;

that expectation will not fail if the test code fails to configure chai to support sinon-chain matchers, and will instead simply be quietly ignored.

On the other hand, if sinon-chai implemented such matchers using a function call syntax, as in:

expect(spy).to.have.been.calledOnce();

then forgetting to do chai.use(require('sinon-chai')) would not cause tests to be quietly ignored and would instead cause them to report an error complaining about undefined not being a callable.

[domenic]

I prefer the shorter syntax.