FromJSON URI use parseURIReference

[seanhess]

Right now, the FromJSON instance for Network.URI.URI doesn't round-trip for relative URIs.

> import Network.URI
> import Data.Aeson 
> let mu :: Maybe URI = parseURIReference "/"
> print mu
Just /

> encode mu
"\"/\""

> decode @URI $ encode mu
Nothing

This PR switches the FromJSON instance to use URI.parseURIReference instead of URI.parseURI. The latter fails with Nothing for relative URIs.