Templates: YAML objects resolve to "true" in conditionals.

Closes #1133.

Note:  If address is a YAML object and you just have $address$
in your template, the word "true" will appear, which may be
unexpected.  (Previously nothing would appear.)

src/Text/Pandoc/Templates.hs

@@ -190,6 +190,7 @@ resolveVar var' val =
        Just (String t)  -> T.stripEnd t
        Just (Number n)  -> T.pack $ show n
        Just (Bool True) -> "true"
+       Just (Object _)  -> "true"
        Just _           -> mempty
        Nothing          -> mempty