[node-fetch] fix Headers

[moroine]

• Links to documentation: https://github.com/node-fetch/node-fetch#class-headers
• Link to GitHub or NPM: https://github.com/node-fetch/node-fetch
• Type of contribution: fix

Other notes:

Pass arguments to cli when using ./quick_run_def_tests.sh so we can do ./quick_run_def_tests.sh --numberOfFlowVersions=1 or even ./quick_run_def_tests.sh --help

[pascalduez]

Ideally this should have been tackled in two different PRs, one for the libdef, one for the CLI. But well, it's Tuesday after all :)