fix: respect tab count in toggle_bufferline()

Change `toggle_bufferline()` fallback value for `vim.o.showtabline` from
0 (always hide tabline) to 1 (show tabline if multiple tabpages exist).
This should fix akinsho#925.

Author: TommyCox

Diff for: lua/bufferline.lua

@@ -90,7 +90,7 @@ end
 local function toggle_bufferline()
   if not config.options.auto_toggle_bufferline then return end
   local item_count = config:is_tabline() and utils.get_tab_count() or utils.get_buf_count()
-  local status = (config.options.always_show_bufferline or item_count > 1) and 2 or 0
+  local status = (config.options.always_show_bufferline or item_count > 1) and 2 or 1
   if vim.o.showtabline ~= status then vim.o.showtabline = status end
 end