fix: ensure header value is a string

[ronag]

I don't think it's something else than a string in any of the branches but the check should still be correct.

[mcollina]

lgtm

[ronag]

So this was not as easy as I thought. We allow other header values than strings. I propose:

• If object, throw
• If null, empty string
• Otherwise stringify with ${val}

[ronag]

@mcollina PTAL

[codecov-commenter]

Codecov Report

Base: 89.85% // Head: 90.43% // Increases project coverage by +0.58% 🎉

Coverage data is based on head (18690a5) compared to base (b06f19f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1899      +/-   ##
==========================================
+ Coverage   89.85%   90.43%   +0.58%     
==========================================
  Files          63       71       +8     
  Lines        5528     6096     +568     
==========================================
+ Hits         4967     5513     +546     
- Misses        561      583      +22     

Impacted Files	Coverage Δ
lib/core/request.js	98.85% <100.00%> (+<0.01%)	⬆️
lib/websocket/frame.js	90.62% <0.00%> (ø)
lib/websocket/websocket.js	92.81% <0.00%> (ø)
lib/websocket/symbols.js	100.00% <0.00%> (ø)
lib/websocket/connection.js	89.24% <0.00%> (ø)
lib/websocket/util.js	86.48% <0.00%> (ø)
lib/websocket/constants.js	100.00% <0.00%> (ø)
lib/websocket/receiver.js	84.51% <0.00%> (ø)
lib/websocket/events.js	100.00% <0.00%> (ø)
lib/fetch/request.js	88.26% <0.00%> (+0.43%)	⬆️
... and 6 more

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[mcollina]

lgtm