make "ht-from-plist" much more convenient for mustache.el

For original `ht-from-plist`, it will convert the plist `(:a 1 :b 2)` into the same symbol constructed hashtable, something like `#s ... (:a 1 :b 2)`, it is not friendly for mustache to render templates.

So, I suggest that when doing the conversion, change the plist's key, e.g. `:a`, into string `"a"`, so mustache will work properly with the generated hashtable.