MapComparator is sensitive to key order

[EnricoMi]

As pointed out in #184 (comment), this example shows that the map comparator identifies a Change where it should flag No change:

val left = Seq((1, """{"foo":"__foo__","bar":"__bar__"}""")).toDF("id", "json")
val right = Seq((1, """{"bar":"__bar__","foo":"__foo__"}""")).toDF("id", "json")
val leftMap = left.select($"id", from_json($"json", MapType(StringType, StringType)))
val rightMap = right.select($"id", from_json($"json", MapType(StringType, StringType)))
val options = DiffOptions.default.withComparator(DiffComparators.map[String, String], MapType(StringType, StringType))
new Differ(options).diff(leftMap, rightMap, "id").show

+----+---+--------------------------------+--------------------------------+
|diff|id |left_entries                    |right_entries                   |
+----+---+--------------------------------+--------------------------------+
|C   |1  |{foo -> __foo__, bar -> __bar__}|{bar -> __bar__, foo -> __foo__}|
+----+---+--------------------------------+--------------------------------+