ref(pkg/chartutil): decrease map lookups

Decrease map lookups in `chartutil.coalesceValues` and move comment to unit
test.

Closes #2663

pkg/chartutil/values.go

@@ -278,17 +278,10 @@ func coalesceValues(c *chart.Chart, v map[string]interface{}) (map[string]interf
 	}
 
 	for key, val := range nv {
-		if _, ok := v[key]; !ok {
-			// If the key is not in v, copy it from nv.
-			v[key] = val
-		} else if ok && v[key] == nil {
-			// When the YAML value is null, we remove the value's key.
-			// This allows Helm's various sources of values (value files or --set) to
-			// remove incompatible keys from any previous chart, file, or set values.
-			// ref: http://www.yaml.org/spec/1.2/spec.html#id2803362
+		if value, ok := v[key]; ok {
+			if value == nil {
 				delete(v, key)
-			continue
-		} else if dest, ok := v[key].(map[string]interface{}); ok {
+			} else if dest, ok := value.(map[string]interface{}); ok {
 				// if v[key] is a table, merge nv's val table into v[key].
 				src, ok := val.(map[string]interface{})
 				if !ok {
@@ -299,6 +292,10 @@ func coalesceValues(c *chart.Chart, v map[string]interface{}) (map[string]interf
 				// values.
 				coalesceTables(dest, src)
 			}
+		} else {
+			// If the key is not in v, copy it from nv.
+			v[key] = val
+		}
 	}
 	return v, nil
 }

pkg/chartutil/values_test.go

@@ -350,6 +350,10 @@ func TestCoalesceValues(t *testing.T) {
 		}
 	}
 
+	// When the YAML value is null, we remove the value's key.
+	// This allows Helm's various sources of values (value files or --set) to
+	// remove incompatible keys from any previous chart, file, or set values.
+	// ref: http://www.yaml.org/spec/1.2/spec.html#id2803362
 	nullKeys := []string{"bottom", "right", "left", "front"}
 	for _, nullKey := range nullKeys {
 		if _, ok := v[nullKey]; ok {