[TT-13257] Fix websocket upgrade with multiple values in Connection header

[Darkness4]

Description

To check for Websocket connection upgrade, replaces the "!=" check with the "not contains" check.

Related Issue

Closes #6449.

How This Has Been Tested

A test case has been added.

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 change)
• [x] Refactoring or add test (improvements in base code or adds test coverage to functionality)

Checklist

• [x] I ensured that the documentation is up to date
• [ ] ~I explained why this PR updates go.mod in detail with reasoning why it's required~
• [ ] ~I would like a code coverage CI quality gate exception and have explained why~

[titpetric]

Hi,

thank you for the contribution. The approach looks good for the most part, two requests raised from review:

• can you use testify/assert in tests (we're standardising on it, but have some fragmentation)
• can you please extend the internal/httputil location with a unit test (ideally that's black box)

Thank you :)

[Darkness4]

@titpetric You can re-review. :slightly_smiling_face: