Document empty array parsing bug for #2079

[PragTob]

Was unsure where to put the specs, there's probably a better
place but here seemed fine for now.

Added some other array related specs for good measure.

Happy to also take a dig at resolving this bug if someone pointed me the right direction :)

For #2079

[grape-bot]

	1 Warning
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

	1 Message
📖	We really appreciate pull requests that demonstrate issues, even without a fix. That said, the next step is to try and fix the failing tests!

Here's an example of a CHANGELOG.md entry:

* [#2080](https://github.com/ruby-grape/grape/pull/2080): Document empty array parsing bug for #2079 - [@PragTob](https://github.com/PragTob).

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[dblock]

Happy to also take a dig at resolving this bug if someone pointed me the right direction :)

Please do. I would look at other array coercing fixes, e.g. #2054 for inspiration.

[PragTob]

Moved to rack/rack#1696 as it seems to me it's rather in rack.

[richkettle]

Im also experiencing this issue. A fix that worked for me is to change

put "/api/url/foo/bar", params: params, headers: headers

to:

put "/api/url/foo/bar", params: params.to_json, headers: headers.merge("CONTENT_TYPE" => "application/json")