Avoid (un)premultiplication for overlay image without alpha channel

Add 'premultiplied' boolean attribute to output info, helps test

lib/composite.js

@@ -8,6 +8,8 @@ const is = require('./is');
  * The overlay image must be the same size or smaller than the processed image.
  * If both `top` and `left` options are provided, they take precedence over `gravity`.
  *
+ * If the overlay image contains an alpha channel then composition with premultiplication will occur.
+ *
  * @example
  * sharp('input.png')
  *   .rotate(180)