fix(res.send): send the bytes covered by a DataView

res.send() recognized a DataView as binary (ArrayBuffer.isView() is
true), but later called Buffer.from(dataView), which produces an empty
Buffer. A non-empty view therefore returned HTTP 200 with
Content-Length: 0 and an empty body.

Root cause

lib/response.js in the object case:

} else if (ArrayBuffer.isView(chunk)) {
  if (!this.get('Content-Type')) {
    this.type('bin');
  }
} else {
  return this.json(chunk);
}

When chunk is a DataView, this sets Content-Type: application/octet-stream
but leaves chunk as the DataView. The length/body handling then falls into:

} else {
  chunk = Buffer.from(chunk, encoding)  // DataView -> empty Buffer
  ...
}

Buffer.from(dataView) yields an empty buffer, so the view's bytes are
silently dropped. The same Buffer.from(typedArray) behavior is correct for
Uint8Array (it materializes the view correctly), but a DataView is not a
typed array — it only references a slice of a parent ArrayBuffer.

Fix

After the existing this.type('bin') guard (which preserves a manually-set
Content-Type), materialise a DataView by its exact byte range instead of
by the DataView itself:

} else if (util.types.isDataView(chunk)) {
  // A DataView references a slice of a parent ArrayBuffer, and
  // Buffer.from(dataView) yields an empty Buffer. Materialise the exact byte
  // range it covers so the response carries the same bytes a Uint8Array would.
  chunk = Buffer.from(chunk.buffer, chunk.byteOffset, chunk.byteLength);
}

This preserves:

• the byte order and the exact range (byteOffset .. byteOffset + byteLength),
• the existing Content-Type when the user set one,
• the ETag when one is configured,
• the Uint8Array path (which already worked via Buffer.from(typedArray)).

Regression tests

New describe block res.send(DataView) in test/res.send.js with 6 cases, all
passing:

• sends the exact byte range covered by the view
• does not override an existing Content-Type
• preserves the ETag when one is set
• sends the bytes of a view that does not cover its whole buffer
• sends cross-realm DataViews unchanged
• does not affect plain ArrayBuffer behaviour

Validation

• npx mocha test/res.send.js → 82 passing (the 6 new DataView tests included)
• npm test → 1267 passing
• npm run lint → clean (no errors)

Related

• Closes res.send(DataView) silently drops the view’s bytes #7510
• Related to the DataView-specific problem discussed in fix(res.send): preserve bytes when sending DataView #7433 (closed without
  merging by its author) and the bare ArrayBuffer support work in res.send(ArrayBuffer) silently sends {} as JSON #7362.