mod_csi_simple: Report whatever's not a stirng and not a stanza

This is either dead code or actually a type error, but catching that
should be the responsibility of the session.send function. This type
check is left since everything after it assumes a stanza object.

These last few commits aren't meant to change any behavior and it did
mark things not stanzas as important, but those would have been mostly
raw strings which are now specially handled.

plugins/mod_csi_simple.lua

@@ -21,7 +21,8 @@ function is_important(stanza) --> boolean, reason: string
 	elseif type(stanza) == "string" then
 		return true, "raw data";
 	elseif not st.is_stanza(stanza) then
-		return true;
+		-- This should probably never happen
+		return true, type(stanza);
 	end
 	if stanza.attr.xmlns ~= nil then
 		-- stream errors, stream management etc