Fix new link format

[hypervillain]

Link format:

const link = {
	"type": "Link.document",
    "value": {
    	"document": { ... }
    }
}

was not correctly matched by Link helper. I added a condition that preserves previous formats.

helpers.url(link, ...)

now correctly returns links instead of empty string.
See this: https://community.prismic.io/t/re-richtext-is-not-transforming-type-paragraph/1452/5 for context