Fix MultiSet#toString and MultiDict#toString

[joshlemer]

Currently:

scala> import scala.collection.immutable._
import scala.collection.immutable._

scala> MultiDict()
res0: scala.collection.immutable.MultiDict[Nothing,Nothing] = Iterable()

scala> MultiDict(1 -> 1, 1 -> 2, 2 -> 2)
res1: scala.collection.immutable.MultiDict[Int,Int] = Iterable((1,1), (1,2), (2,2))

scala> MultiSet()
res2: scala.collection.immutable.MultiSet[Nothing] = Iterable()

scala> MultiSet(1,1,1,2,2)

With this PR:

scala> import scala.collection.immutable._
import scala.collection.immutable._

scala> MultiDict()
res0: scala.collection.immutable.MultiDict[Nothing,Nothing] = MultiDict()

scala> MultiDict(1 -> 1, 1 -> 2, 2 -> 2)
res1: scala.collection.immutable.MultiDict[Int,Int] = MultiDict(1 -> 1, 1 -> 2, 2 -> 2)

scala> MultiSet()
res2: scala.collection.immutable.MultiSet[Nothing] = MultiSet()

scala> MultiSet(1,1,1,2,2)
res3: scala.collection.immutable.MultiSet[Int] = MultiSet(1, 1, 1, 2, 2)