fix: excel extractor header detection bug

[carlbrugger]

Please explain how to summarize this PR for the Changelog:

This release fixes a bug in @flatfile/plugin-excel-extractor when using the explicitHeaders header detection algorithm.

Tell code reviewer how and what to test:

Extraction should not error when using the explicitHeaders algorithm

listener.use(
  ExcelExtractor({
    headerDetectionOptions: {
      algorithm: 'explicitHeaders',
      headers: ['fiRsT NamE', 'LaSt nAme', 'emAil'],
    },
  })
)