Tab at the end of line is not working for default config

[glyh]

Checkhealth

Put result of :checkhealth compe.

## compe:snippet
  - OK: snippet engine detected.

## compe:mapping
  - INFO: `compe#complete` is not mapped
  - INFO: `compe#confirm` is not mapped
  - INFO: `compe#close` is not mapped
  - INFO: `compe#scroll` is not mapped

Describe the bug

To Reproduce

Steps to reproduce the behavior with the minimal config:

1. write some string, end with a none empty character( for example, "a")
2. put cursor at the end of line in insert mode.
3. press tab, nothing happen

Actual behavior

No '\t' appeared

Expected behavior

Add a new '\t'

Minimal vimrc

require("packer").startup({function() 
  use "wbthomason/packer.nvim"
  use "hrsh7th/nvim-compe"
end,config = {
  git = {default_url_format = "https://hub.fastgit.org/%s"}
}})

require'compe'.setup {
  enabled = true;
}
local t = function(str)
  return vim.api.nvim_replace_termcodes(str, true, true, true)
end

local check_back_space = function()
    local col = vim.fn.col('.') - 1
    return col == 0 or vim.fn.getline('.'):sub(col, col):match('%s') ~= nil
end

_G.tab_complete = function()
  if vim.fn.pumvisible() == 1 then
    return t "<C-n>"
  elseif check_back_space() then
    return t "<Tab>"
  else
    return vim.fn['compe#complete']()
  end
end

vim.api.nvim_set_keymap("i", "<Tab>", "v:lua.tab_complete()", {expr = true})

[stale]

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