Fix Firefox connector breakage after 69ab4b0b1
Fix Firefox connector breakage after 69ab4b0b1

This also makes a few changes to `callMethod`:

- Removes the deprecation warning when a string is passed and removes the
  `httpMethod` `options` property. Any call that passes a string and includes
  `data` can just remain as a POST. If `data` is null or undefined, a `GET`
  will be sent. (If we find ourselves needing other methods, we can reevaluate.)
- Renames `httpHeaders` to `headers` for consistency with other code (and in the
  absence of `httpMethod`)

gracile-fr/zotero,gracile-fr/zotero,gracile-fr/zotero