add fast path for unit conversion

Author: ajungg

core/src/androidTest/java/io/snabble/sdk/UnitTest.java

@@ -28,6 +28,7 @@ public void testUnitPrices() throws IOException, Snabble.SnabbleException {
         Assert.assertEquals(199, test_cm_1.getPrice());
         Assert.assertEquals(1990, test_cm_1.getPriceForQuantity(1000, RoundingMode.HALF_UP));
         Assert.assertEquals(498, test_cm_1.getPriceForQuantity(250, RoundingMode.HALF_UP));
+        Assert.assertEquals(497, test_cm_1.getPriceForQuantity(250, RoundingMode.FLOOR));
     }
 
     public void testUnits() {

core/src/main/java/io/snabble/sdk/Unit.java

@@ -51,6 +51,8 @@ public static Unit fromString(String value) {
     }
 
     public static BigDecimal convert(BigDecimal value, Unit from, Unit to, int scale, RoundingMode rm) {
+        if (from == to) return value;
+
         value = value.setScale(scale, rm);
 
         if (from == LITER && to == DECILITER) return value.divide(new BigDecimal(10), rm);