Add assert that fields passed into FieldsParam.fields are an array or a function

[MacMcIrish]

This line can also be removed, and just return the result, consider an assert that the result of the function is a list?

lambda-serverless-api/src/params/fields-param.js

Line 13 in 5ff6809

return typeof result === 'string' ? objectFields.split(result) : result;