Maintain singleton sequence of array

[andrew-coleman]

The [] syntax in a path expression should output the result sequence as an array even if the sequence contains one value (singleton sequence). However, this is not happening if that value is an array. Instead it's just using that as the result array. This PR fixes that, so that it will be enclosed in another array.

Resolves #399

Signed-off-by: andrew-coleman andrew_coleman@uk.ibm.com

[coveralls]

Coverage remained the same at 100.0% when pulling 47840e4 on singleton_array into 34c2a09 on master.

[mattbaileyuk]

For later reference, with data:

{"epochSeconds":1578381600,"value": 3}

the expression $.[value,epochSeconds][] current produces

[3, 1578381600]

when it should (and this PR will enable it to) produce

[
      [3, 1578381600]      
]