Compare ignoring nulls in circe.

Consider the following.
```
Json did not match "{
  "a" : "01",
  "b" : null,
  "c" : "1"
}" did not match "{
 "a": "01",
 "c": "1"
 }"

Json Diff:
"[
  {
    "op" : "remove",
    "path" : "/b"
  }
]"
```