DataFormat is not respected when saving images with a filename

[azurefx]

There are two save_ functions in ImageMagick.jl which accept AbstractString and Stream.
The Stream version calls getblob and eventually calls MagickSetImageFormat, so we can use Stream(format"XXX",filename) to specify the file format regardless of file extension. The AbstractString version calls writeimage(wand, filename) directly and lets ImageMagick to infer the format.
Thus if we call save(File(format"JPEG","xxx.png"),img), we get a PNG-encoded file (which is not expected).

[timholy]

PR welcome!