add currency in toString method

Author: otaviojava

src/main/java/org/javamoney/moneta/function/MonetarySummaryStatistics.java

@@ -137,7 +137,8 @@ public MonetaryAmount getAverage() {
 	@Override
 	public String toString() {
 		StringBuilder sb = new StringBuilder();
-		sb.append("[count:").append(count).append(",");
+		sb.append("[currency: ").append(empty.getCurrency()).append(",");
+		sb.append("count:").append(count).append(",");
 		sb.append("min:").append(min).append(",");
 		sb.append("max:").append(max).append(",");
 		sb.append("sum:").append(sum).append(",");