go.blog/cmd/blog: fix JSON content-type
go.blog/cmd/blog: fix JSON content-type

R=dsymonds
CC=golang-dev
https://codereview.appspot.com/9302044

d0f/go-zh.blog,bryanxu/go-zh.blog,webfd/go-zh.blog,bryanxu/go-zh.blog,d0f/go-zh.blog,webfd/go-zh.blog