deps: use extend2 instead of extend

[dead-horse]

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

[codecov]

Codecov Report

Merging #674 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #674   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          26     26           
  Lines         638    638           
=====================================
  Hits          638    638

Impacted Files	Coverage Δ
lib/egg.js	100% <100%> (ø)	⬆️

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