修改MJRefreshNormalHeader.arrowView的renderingMode为AlwaysTemplate，使得可以更改下拉箭头图片的颜色

MJRefresh/Custom/Header/MJRefreshNormalHeader.m

@@ -21,7 +21,7 @@ - (UIImageView *)arrowView
 {
     if (!_arrowView) {
         UIImage *image = [UIImage imageNamed:MJRefreshSrcName(@"arrow.png")] ?: [UIImage imageNamed:MJRefreshFrameworkSrcName(@"arrow.png")];
-        UIImageView *arrowView = [[UIImageView alloc] initWithImage:image];
+        UIImageView *arrowView = [[UIImageView alloc] initWithImage:[image imageWithRenderingMode:(UIImageRenderingModeAlwaysTemplate)]];
         [self addSubview:_arrowView = arrowView];
     }
     return _arrowView;