Adding a compare function to be used in expected object template

[sergejsha]

The idea of this PR is to add a possibility of using a compare function inside the expected object. This allows not only checking for static values, but also adding more sophisticated checks like check for being not null, check for been defined etc.

Here is an example. I store an object and then additionally check that id, createdAt and updatedAt properties are present in the object. The check would not be possible without the compare function because returned values are expected to be different between different test runs.

        it('should store object', async function () {
            const publisherData = {
                name: 'publisher',
                descr: 'description',
                urls: { homepage: 'http://homepage' }
            }
            const publisher = await repository.findOrCreatePublisher(publisherData)
            expect(publisher).containSubset({
                ...publisherData,
                id: id => id,
                createdAt: createdAt => createdAt,
                updatedAt: updatedAt => updatedAt
            })
        })

This might also solve #64.

[eagleeye]

@beworker not bad =) Please add some examples to the README

[sergejsha]

Here we are. Some examples were added.

[eagleeye]

@beworker thanks a lot!

[sergejsha]

Thanks you for releasing! Now I can remove locally modified chai-subset copy :)

[elmigranto]

This breaks comparing functions (see #68) or throws on class instances.

const int = (left, right) => left - right
class Expected {}

expect({comparator: int, ctor: Expected}).to.containSubset({
  comparator: int, // fails, even though function is the same
  ctor: Expected,  // throws TypeError
})

// TypeError: Class constructor Expected cannot be invoked without 'new'
//      at <redacted>/node_modules/chai-subset/lib/chai-subset.js:75:13
//      at Array.every (<anonymous>)
//      at compare (node_modules/chai-subset/lib/chai-subset.js:68:33)
//      at Proxy.<anonymous> (node_modules/chai-subset/lib/chai-subset.js:21:5)
//      <redacted>

@beworker @eagleeye