Signals: always include headers object

[silesky]

• add onReadyStateChange
• always include headers

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 0e30c7ce853ea49bd618b6975444720936f9bded

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | -------------------------- | ----- | | @segment/analytics-signals | Patch |

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