Fix stringify for bracket mode with arrays containing null

[dubzzz]

I ran into an unexpected behaviour while running property based tests against query-string.

The unexpected behaviour is the following: when the settings bracket is enabled, null values contained inside arrays are badly exported causing problems on parsing side

m.stringify({bar: ['a', null, 'b']}, {arrayFormat: 'bracket'}) //=> "bar[]=a&bar&bar[]=b"
m.parse('bar[]=a&bar&bar[]=b', {arrayFormat: 'bracket'}) //=> {bar: [null, 'b']}

The commit contains both:

• the fix to serialize arrays correctly when enabling bracket mode
• the unit tests to assess the behaviour is correct
• the original property based test that detected the problem

[dubzzz]

@sindresorhus
I believe that it would be great to fix this issue. Fix in this very same PR, #138.
Without the fix parsing fails to read some query params it has itself generated.

Thanks in advance for your feedback

[sindresorhus]

Good catch. Thank you! :)

[sindresorhus]

Any idea why the fastcheck test fails on Node.js 10? https://travis-ci.org/sindresorhus/query-string/jobs/374050952

[dubzzz]

I am gonna have a look to it. Seems that it encountered a counterexample.
I let you know as soon as I get more details.

[dubzzz]

Submitting a fix for this issue as soon as possible.
In my original test, I used the wrong type for the sort parameter.

[dubzzz]

In the log we can see that the failing case is:

• obj = {}
• opts = {"arrayFormat":"bracket","strict":false,"encode":false,"sort":true}

And that it caused the following error: TypeError: The comparison function must be either a function or undefined in sort