Monoid[JsObject] does not satisfy associative law

(Moved from playframework/playframework#4651)

Welcome to Scala version 2.11.6 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_45).
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scala> import play.api.libs.json._
import play.api.libs.json._

scala> import play.api.libs.json.Reads.JsObjectMonoid
import play.api.libs.json.Reads.JsObjectMonoid

scala> val a = Json.obj("key" -> Json.obj("x" -> 1))
a: play.api.libs.json.JsObject = {"key":{"x":1}}

scala> val b = Json.obj("key" -> 2)
b: play.api.libs.json.JsObject = {"key":2}

scala> val c = Json.obj("key" -> Json.obj("y" -> 3))
c: play.api.libs.json.JsObject = {"key":{"y":3}}

scala> JsObjectMonoid.append(JsObjectMonoid.append(a, b), c)
res0: play.api.libs.json.JsObject = {"key":{"y":3}}

scala> JsObjectMonoid.append(a, JsObjectMonoid.append(b, c))
res1: play.api.libs.json.JsObject = {"key":{"x":1,"y":3}}

libraryDependencies += "com.typesafe.play" %% "play-json" % "2.4.0"

scalaVersion := "2.11.6"

Start by reproducing the documented Scala REPL expressions using Reads.JsObjectMonoid.append with the three JsObject values shown. Inspect the JsObjectMonoid implementation and existing law or JSON-object tests, if present. Done should include behavior that satisfies the associative law for this example and a regression test covering it.