httplib: fix the header function for User-Agent

@@ -355,7 +355,7 @@ func (b *BeegoHttpRequest) getResponse() (*http.Response, error) {
 		Jar:       jar,
 	}
 
-	if b.setting.UserAgent != "" {
+	if b.setting.UserAgent != "" && b.req.Header.Get("User-Agent") == "" {
 		b.req.Header.Set("User-Agent", b.setting.UserAgent)
 	}
 

@@ -203,3 +203,13 @@ func TestToFile(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestHeader(t *testing.T) {
+	req := Get("http://httpbin.org/headers")
+	req.Header("User-Agent", "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.57 Safari/537.36")
+	str, err := req.String()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Log(str)
+}