build_runner_session.rb

Workhorse was not able to parse the `headers` property since it was expecting a
key => array as a structure.

Error from workhorse: preAuthorizeHandler: decode authorization response: json:
cannot unmarshal string into Go struct field TerminalSettings.Header of type
[]string

Workhorse was not changed since this api is already used for the environment
terminals.

gitlab-org/gitlab-ce#25990