save buffer to PNG format instead

[dangkhoasdc]

If the input image is a PNG file with the 'Alpha' channel, PIL could not save it as "JPG" format:

OSError: cannot write mode RGBA as JPEG

(Source: https://stackoverflow.com/questions/48248405/cannot-write-mode-rgba-as-jpeg )

To fix it, we can change the format of buffer when saving to 'PNG' instead. It can handle both 3-channel and 4-channel images. It also aligns with the format when we encode to base64 string.

[blackary]

Thanks!

[blackary]

Added a demo page -- works great!