Images not showing on Twitter cards or Open Graph

[Jubijub]

Expected Behavior

• Featured images (using the featured = xxxx in the front matter should show up as Twitter cards or Open Graph pictures
• this is the expected behaviour because the default embedded template https://github.com/gohugoio/hugo/blob/master/tpl/tplimpl/template_embedded.go has the code to show <meta name="twitter:image" (line 236 and beyond) leveraging image or featured parameters

Current Behavior

• No image are showing (I am no Go expert, but checking at the generated code, no <meta name="twitter:image" shows up
• Twitter card validator logically picks no images either.

Possible Solution

• I followed this approach and it works, but it's not very clean

Steps to Reproduce or Template with Error (for bugs)

1. create a post with a featured image
2. notice the generate code contains no twitter card image tag

Your Environment

• site is hosted on Gitlab pages, but is equally reproducible locally using hugo server

Hugo Version:

Hugo Static Site Generator v0.37.1 linux/amd64 BuildDate: 
GOOS="linux"
GOARCH="amd64"
GOVERSION="go1.10"

Browser:
Any, the problem is in the generated HTML

Desktop or Mobile:
Any

Link to your project:
https://www.jubi.blog/blog/2018-03-why-hiring-can-seem-unfair/

[pacollins]

#115 addressed this for the site as a whole, but there is not currently OG for posts. If you'd like to create a PR, I'm sure everyone would really appreciate it!