fix golint issues (#458)

core/trace/carrier.go

@@ -30,9 +30,9 @@ func (h httpCarrier) Set(key, val string) {
 func (g grpcCarrier) Get(key string) string {
 	if vals, ok := g[strings.ToLower(key)]; ok && len(vals) > 0 {
 		return vals[0]
-	} else {
-		return ""
 	}
+
+	return ""
 }
 
 func (g grpcCarrier) Set(key, val string) {

core/trace/propagator.go

@@ -27,35 +27,35 @@ type (
 )
 
 func (h httpPropagator) Extract(carrier interface{}) (Carrier, error) {
-	if c, ok := carrier.(http.Header); !ok {
-		return nil, ErrInvalidCarrier
-	} else {
+	if c, ok := carrier.(http.Header); ok {
 		return httpCarrier(c), nil
 	}
+
+	return nil, ErrInvalidCarrier
 }
 
 func (h httpPropagator) Inject(carrier interface{}) (Carrier, error) {
 	if c, ok := carrier.(http.Header); ok {
 		return httpCarrier(c), nil
-	} else {
-		return nil, ErrInvalidCarrier
 	}
+
+	return nil, ErrInvalidCarrier
 }
 
 func (g grpcPropagator) Extract(carrier interface{}) (Carrier, error) {
 	if c, ok := carrier.(metadata.MD); ok {
 		return grpcCarrier(c), nil
-	} else {
-		return nil, ErrInvalidCarrier
 	}
+
+	return nil, ErrInvalidCarrier
 }
 
 func (g grpcPropagator) Inject(carrier interface{}) (Carrier, error) {
 	if c, ok := carrier.(metadata.MD); ok {
 		return grpcCarrier(c), nil
-	} else {
-		return nil, ErrInvalidCarrier
 	}
+
+	return nil, ErrInvalidCarrier
 }
 
 func Extract(format, carrier interface{}) (Carrier, error) {