Issue 140: Fix call to add_headers of api_get_impl()

[konradoberwimmer]

Please ensure the following before submitting a PR:

• if suggesting code changes or improvements, open an issue first
• for all but trivial changes (e.g., typo fixes), add your name to DESCRIPTION
• for all but trivial changes (e.g., typo fixes), documentation your change in NEWS.md with a parenthetical reference to the issue number being addressed
• [-] if changing documentation, edit files in /R not /man and run devtools::document() to update documentation
• [IGNORED, no similar tests using API token] add code or new test files to /tests for any new functionality or bug fix
• make sure R CMD check runs without error before submitting the PR