Fix luacheck

Fix luacheck warnings in spam.lua

Fix global variable initialization (was needed only locally)

Add beerchat to luacheck

.luacheckrc

@@ -5,7 +5,8 @@ globals = {
 read_globals = {
 	-- Stdlib
 	string = {fields = {"split"}},
-	table = {fields = {"copy", "getn"}},
+	table = {fields = {"copy", "getn", "indexof"}},
+	beerchat = {fields = {"has_player_muted_player", "execute_callbacks"}},
 
 	-- Minetest
 	"minetest",

util/spam.lua

@@ -28,7 +28,7 @@ local function words_ratio(str, ratio)
 end
 
 function mail.check_spam(message)
-    spam_checks = {}
+    local spam_checks = {}
     if caps_ratio(message.subject) == 1 or caps_ratio(message.body) > 0.4 then
         table.insert(spam_checks, "caps")
     end