Avoid coercing parameter with multiple types to an empty Array

[stanhu]

If an optional parameter has multiple types (e.g. [File, String]) and a
nil parameter is passed, Grape would previously coerce the value to an
empty Array. This can break certain APIs that treat nil values
differently from an empty Array.

To fix this, we refactor the nil handling into a coerce_nil method and
remove the special case check for handling an Array of types.

Closes #2018

[stanhu]

I'm not sure about this type.is_a?(Array) check. I assume that passing a nil value to a parameter with multiple types might lead to ambiguity?

[ericproulx]

Unfortunately, it was necessary

    it 'handles default value to array' do
      subject.params do
        optional :a, type: Array[Integer], default: []
      end
      subject.put('/test') { declared(params).to_json }
      put '/test', { a: nil }
      expect(last_response.status).to eq(200)
      expect(last_response.body).to eq('{"a":[]}')
    end

That test fails because type != array but it's type.is_a?(Array)
I'll make a PR to fix it

[dblock]

Looks like a bug, thanks for the fix! Move the changelog entry and I'll merge.

[dblock]

See rubocop offense, I'll merge on green.

[dblock]

just val

[dblock]

this is a good place for a case

case type
when Array
when ...

[stanhu]

That doesn't actually work in this case because we need to distinguish between type.is_a?(Array) and type == Array (https://stackoverflow.com/questions/3908380/ruby-class-types-and-case-statements).