collapse shortcode content can write formative code and change line (#561)

Author: drizzle

layouts/shortcodes/collapse.html

@@ -1,12 +1,8 @@
-{{ if .Get "content" }}
 {{ if .Get "summary" }}
 {{ else }}
 {{ warnf "missing value for param 'summary': %s" .Position }}
 {{ end }}
-{{ else }}
-{{ errorf "missing value for param 'content': %s" .Position }}
-{{ end }}
 <p><details {{ if (eq (.Get "openByDefault") true) }} open=true {{ end }}>
   <summary markdown="span">{{ .Get "summary" | markdownify }}</summary>
-    {{ .Get "content" | markdownify }}
+  {{ .Inner | markdownify }}
 </details></p>