add PNG file to fixtures as an example binary file

Downloaded from http://commons.wikimedia.org/wiki/File:White_jk_r.svg
Image is public domain via the Creative Commons CC0 1.0 Universal Public Domain Dedication.
Image author: Nicu Buculei
Part of the Openclipart collection
replace example PNG file so that there will be a diff of a binary file

Downloaded from http://commons.wikimedia.org/wiki/File:White_jk_b.svg
Image is public domain via the Creative Commons CC0 1.0 Universal Public Domain Dedication.
Image author: Nicu Buculei
Part of the Openclipart collection
add test of .diff endpoint on a binary file
add tests for binary blobs & fix name of key in blob JSON