fix: properly merge headers in fetchWithEvent

[brc-dd]

fixes nuxt/nuxt#31073 (will need backporting to v1 to actually fix though)

Currently, the merging behavior is inconsistent between useRequestFetch and client $fetch because of spreading headers without normalization. This PR fixes that by doing essentially same as what's done above in that file:

h3/src/utils/proxy.ts

Lines 37 to 42 in 0a796ff

	// Headers
	const fetchHeaders = mergeHeaders(
	getProxyRequestHeaders(event, { host: target.startsWith("/") }),
	opts.fetchOptions?.headers,
	opts.headers,
	);

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 79.36%. Comparing base (a58d7c9) to head (f70b39a).
Report is 224 commits behind head on main.

Files with missing lines	Patch %	Lines
src/utils/proxy.ts	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #980      +/-   ##
==========================================
+ Coverage   77.83%   79.36%   +1.53%     
==========================================
  Files          47       43       -4     
  Lines        4286     3296     -990     
  Branches      611      795     +184     
==========================================
- Hits         3336     2616     -720     
+ Misses        933      677     -256     
+ Partials       17        3      -14     

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