fix: preserve headers from Request input

[yusuke99]

Closes #90

Changes

Updated mergeHeaders function to include headers from the provided Request object.

Testing

Added test to verify that headers are preserved when sending a request by passing Request object.

Note

You can also test by linking the fixed package and running the reproduction code mentioned in issue.

Link the local package instead of from the registry 👇

package.json

"dependencies": {
  "up-fetch": "link:/path/to/up-fetch"
}