DownloadsBackend inconsistently closes streams

[omus]

The following test case is from #470:

@testset "issue 466" begin
    file_name = "hang.txt"

    try
        S3.put_object("anewbucket", file_name)

        # Note: Using `eof` for these tests can hang when using an unclosed `Base.BufferStream`

        stream = S3.get_object("anewbucket", file_name, Dict("return_stream" => true))
        if AWS.DEFAULT_BACKEND[] isa AWS.HTTPBackend
            @test !isopen(stream)
        else
            @test isopen(stream)
        end

        stream = Base.BufferStream()
        S3.get_object("anewbucket", file_name, Dict("response_stream" => stream))
        if AWS.DEFAULT_BACKEND[] isa AWS.HTTPBackend
            @test !isopen(stream)
        else
            @test_broken isopen(stream)
        end

        stream = Base.BufferStream()
        S3.get_object(
            "anewbucket",
            file_name,
            Dict("response_stream" => stream, "return_stream" => true),
        )
        if AWS.DEFAULT_BACKEND[] isa AWS.HTTPBackend
            @test !isopen(stream)
        else
            @test isopen(stream)
        end
    finally
        S3.delete_object("anewbucket", file_name)
    end
end

Note how when using a non-IOBuffer and not using the "return_stream" parameter that the I/O stream is closed but in all other cases it leaves the stream open. This inconsistency should not be present.

Additionally, most likely the behaviour of the HTTPBackend and the DownloadsBackend should made to be consistent.

[omus]

Related to #433

[ericphanson]

Just to add another ref, I think this is case 2 from #396 (comment)

[omus]

As of AWS.jl 1.63.0 (#384) streams are no longer closed when using the feature use_response_type (e.g. @service S3 use_response_type=true). If you enable this feature and pass in a Base.BufferStream you're responsible for closing it yourself.

When not enabling the use_response_type feature the current behaviour as of AWS 1.62.1 remains unchanged.