Added some test coverage for the String method

ostat_test.go

@@ -1,6 +1,7 @@
 package ostat
 
 import (
+	"fmt"
 	"math"
 	"testing"
 )
@@ -267,3 +268,14 @@ func TestMidStreamStatWithData(t *testing.T) {
 		t.Errorf("max: %f != %f != %f", mss.Max, mss.Max, max2)
 	}
 }
+
+func TestString(t *testing.T) {
+	ps := NewPopulationStat()
+	for _, v := range []float64{1, 1, 2, 2} {
+		ps.Push(v)
+	}
+	expected := "{n:4 min:1 max:2 mean:1.5 variance:0.25 stdDev:0.5}"
+	if fmt.Sprint(ps) != expected {
+		t.Errorf("failed to generate correct string")
+	}
+}