Made lume.reduce check initial value for nil instead of non-truthy

Fixes #32

lume.lua

@@ -286,8 +286,8 @@ end
 
 
 function lume.reduce(t, fn, first)
+  local started = first ~= nil
   local acc = first
-  local started = first and true or false
   local iter = getiter(t)
   for _, v in iter(t) do
     if started then