test(helper/proxy): does not propagate undefined request headers

[BarryThePenguin]

• Adds a failing test using the documentation as an example
• Remove undefined headers from ProxyRequestInit
• Handle duplicate headers from Request and ProxyRequestInit

Fixes #3920

[codecov]

Codecov Report

Attention: Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.31%. Comparing base (e7fbc3e) to head (d52bde4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/helper/proxy/index.ts	95.65%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3921   +/-   ##
=======================================
  Coverage   91.31%   91.31%           
=======================================
  Files         168      168           
  Lines       10673    10686   +13     
  Branches     3167     3077   -90     
=======================================
+ Hits         9746     9758   +12     
- Misses        926      927    +1     
  Partials        1        1           

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[BarryThePenguin]

Re: #3919, my basic understanding of the spread operator is that new objects are created and all the keys are copied. This avoids that extra overhead

[BarryThePenguin]

This is what lines 94 and 97 were doing previously..

hono/src/helper/proxy/index.ts

Lines 92 to 99 in e7fbc3e

	const req = new Request(
	input,
	// @ts-expect-error `headers` in `requestInit` is not compatible with HeadersInit
	{
	...buildRequestInitFromRequest(raw),
	...requestInit,
	}
	)

[BarryThePenguin]

Previously the headers from buildRequestInitFromRequest(raw) were being overwritten by ...requestInit on line 97

hono/src/helper/proxy/index.ts

Lines 95 to 98 in e7fbc3e

	{
	...buildRequestInitFromRequest(raw),
	...requestInit,
	}

[BarryThePenguin]

Unsure if this is right..

It's deleting a header when the value is undefined, otherwise the value is appended. This means values in both the ProxyRequestInit and the request will be merged, rather than the value from the request overwriting the value from ProxyRequestInit, for example.

[BarryThePenguin]

Duplicate request headers using Header#append. Alternatively, using Header#set

[BarryThePenguin]

Looking at this again, maybe this should actually be..

Suggested change

	method: request.method,
	body: request.body,
	duplex: request.body ? 'half' : undefined,
	...requestInit,
	headers,
	...requestInit,
	method: request.method,
	body: request.body,
	duplex: request.body ? 'half' : undefined,
	headers,

..to avoid requestInit overwriting any of method, body, duplex and headers?