Added clear() methods to JSONObject and JSONArray

[stranck]

I noticed that there wasn't this easy way to clear all the childs of a JSONObject or a JSONArray, so I added it.
Changes in code are really simple

[stleary]

@stranck Thanks for submitting this PR. The submitter is usually asked to provide a use case or justification, but I guess in this case it is self-evident. No objection to proceeding after you add the appropriate unit tests.

[stranck]

I should've added them :)
Tell me if they're alright and thank you for being so fast in replying ahahahhaha

[stleary]

What problem does this code solve?
This is an enhancement to include a missed clear() method, not a bug fix. The API includes JSONObject and JSONArray methods that recognize they are containers, so adding this method makes sense.

Risks
Low

Changes to the API?
New public methods are added to JSONObject and JSONArray

Will this require a new release?
No

Should the documentation be updated?
No

Does it break the unit tests?
No, new unit tests were added

Was any code refactored in this commit?
No

Review status
APPROVED

[stleary]

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