Sass map strings are not using quoted keys

[alexanderflink]

When rendering a sass string from a javascript object, the keys are not being put in quotes. This causes problems if the keys have names such as red or black. Here's an example:

const test = {
  key1: 'hej',
  key2: 'hej',
}

const sassMap = sassUtils.castToSass(test)
console.log(sassUtils.sassString(sassMap)) // (key1: hej, key2: hej)

It would be better if they keys were in quotes, as suggested by the Sass docs.