image/jpeg: encoding with RGB profile causing loss of image saturation

[kiwionly]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

1.9.2

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

windows 10

What did you do?

I encoding a JPEG file with RGB profile, some how the encoding result look like not correct, I using gimp and it compress correctly, the output result show a bit darker then the original image.

here the code from go playground

Original image

After encode

The images diff

What did you expect to see?

What did you see instead?

[odeke-em]

/cc @nigeltao @fogleman