Add flag for user-agent

add option to set/unset user-agent header (#96)

* add option to set/unset user-agent header

* add unit test and fix indentation

* update test for user-agent

* update indentation

* update description of the field

* fix typo

* refactor assign headers

* update version add github action npm-publish

* update version in package-lock.json

* upgrade moment to 2.29.2