fix: aggregate array request header values like a real HTTP server

[LeSingh1]

Problem

inject() accepts array-valued request headers (allowed by the http.OutgoingHttpHeaders input type) to simulate the same header being sent multiple times. However, array values were serialized with a bare '' + value coercion, which does not match how a real Node.js HTTP server aggregates repeated header lines onto request.headers / request.rawHeaders.

Given:

inject(dispatch, {
  method: 'GET',
  url: '/',
  headers: { baz: ['first', 'second'], 'set-cookie': ['a=1', 'b=2'], cookie: ['x=1', 'y=2'] }
})

	light-my-request (before)	Real Node.js HTTP server
headers.baz	"first,second"	"first, second"
headers['set-cookie']	"a=1,b=2"	["a=1", "b=2"]
headers.cookie	"x=1,y=2"	"x=1; y=2"
rawHeaders	["baz","first,second", ...]	["baz","first","baz","second", ...]

The Node behavior in the right column was verified empirically against http.createServer receiving duplicate header lines over a raw socket. See issue #264.

Fix

Aggregate array header values exactly as http.IncomingMessage does (per the message.headers docs):

• set-cookie is kept as an array of its individual values.
• cookie values are joined with '; '.
• Duplicates of single-value headers (content-type, host, authorization, …) are discarded, keeping the first.
• All other headers are joined with ', '.
• Every supplied value is preserved as a separate rawHeaders entry.

Non-array (string/number) header values are unchanged, so existing behavior is fully preserved for the common case. One existing test that asserted the old bare-comma output ('1,two,3') was updated to the Node-faithful '1, two, 3'.

Verification

• Added regression tests covering the ', ' join + rawHeaders expansion, set-cookie array preservation, cookie '; ' join, single-value-header duplicate discarding, and the undefined-inside-array assertion.
• npm test passes: lint, 172 unit tests at 100% coverage (c8 --100), and 54 tstyche type assertions.

Closes #264

[mcollina]

lgtm