layouts: Remove quoting of structured data

I don't understand it, but quoting these values causes Hugo to use
escape codes for non-ASCII (?) characters. For example:

  "description": "A thriving oasis in Jordan\x27s desert is at risk of disappearing."

Google's Search Console shows the following error:

    Unparsable structured data > Bad escape sequence in string

We had a similar issue with dates that was fixed in #111 and the
solution is the same here.

layouts/_default/single.html

@@ -38,12 +38,12 @@
   }
   {{- if or (.Params.keywords) (or (.Params.categories) (.Params.tags)) -}}
   ,
-  "keywords": "{{ delimit (union .Params.keywords (union .Params.categories .Params.tags)) ", " }}"
+  "keywords": {{ delimit (union .Params.keywords (union .Params.categories .Params.tags)) ", " }}
   {{- end }}
 
   {{- with .Params.description -}}
   ,
-  "description": "{{ . }}"
+  "description": {{ . }}
   {{- end }}
 }
 </script>