[#43] generator: Fix initial payload generation

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

internal/datagen/generator.go

@@ -40,11 +40,15 @@ func NewGenerator(vu modules.VU, size int) Generator {
 	if size <= 0 {
 		panic("size should be positive")
 	}
+
+	r := rand.New(rand.NewSource(time.Now().UnixNano()))
+	buf := make([]byte, size+TailSize)
+	r.Read(buf)
 	return Generator{
 		vu:   vu,
 		size: size,
-		rand: rand.New(rand.NewSource(time.Now().UnixNano())),
-		buf:  make([]byte, size+TailSize),
+		rand: r,
+		buf:  buf,
 	}
 }