fix: upgradeing to websocket caused server crashed (closes #777)

[whatwewant]

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Description

Motivation and Context

fix https://github.com/chimurai/http-proxy-middleware/issues/777

How has this been tested?

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.