Added String() to MapSet

Signed-off-by: Tim Henderson <[email protected]>

Author: timtadh

Diff for: set/mapset.go

@@ -1,5 +1,9 @@
 package set
 
+import (
+	"fmt"
+)
+
 import (
 	"github.com/timtadh/data-structures/errors"
 	"github.com/timtadh/data-structures/types"
@@ -162,3 +166,7 @@ func (m *MapSet) ProperSuperset(b types.Set) bool {
 	return ProperSuperset(m, b)
 }
 
+func (m *MapSet) String() string {
+	return fmt.Sprintf("%v", m.Set)
+}
+