add middleware module type assertion

[bung87]

Checklist

• [ ] npm test passes
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [ ] commit message follows commit guidelines

Affected core subsystem(s)

Description of change

ensure middleware export function

[codecov[bot]]

Codecov Report

Merging #241 (fe4c098) into master (91c04cc) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #241   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files          19       19           
  Lines        1181     1182    +1     
  Branches      207      207           
=======================================
+ Hits         1178     1179    +1     
  Misses          3        3           

Impacted Files	Coverage Δ
lib/loader/mixin/middleware.js	96.77% <100.00%> (+0.05%)	:arrow_up:

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[fengmk2]

still missing the test case

[bung87]

sorry for that, I tried at that time, but fails, now I dont use nodejs anymore. I dont mind close this or what ever.