Use DefaultWriter and DefaultErrorWriter for debug messages (#1891)

Aligns behaviour according to documentation.

debug.go

@@ -8,7 +8,6 @@ import (
 	"bytes"
 	"fmt"
 	"html/template"
-	"os"
 	"runtime"
 	"strconv"
 	"strings"
@@ -54,7 +53,7 @@ func debugPrint(format string, values ...interface{}) {
 		if !strings.HasSuffix(format, "\n") {
 			format += "\n"
 		}
-		fmt.Fprintf(os.Stderr, "[GIN-debug] "+format, values...)
+		fmt.Fprintf(DefaultWriter, "[GIN-debug] "+format, values...)
 	}
 }
 
@@ -98,6 +97,8 @@ at initialization. ie. before any route is registered or the router is listening
 
 func debugPrintError(err error) {
 	if err != nil {
-		debugPrint("[ERROR] %v\n", err)
+		if IsDebugging() {
+			fmt.Fprintf(DefaultErrorWriter, "[GIN-debug] [ERROR] %v\n", err)
+		}
 	}
 }