Fix authorization for web terminals

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

app/models/ci/build_runner_session.rb

@@ -17,7 +17,7 @@ module Ci
       {
         subprotocols: ['terminal.gitlab.com'].freeze,
         url: "#{url}/exec".sub("https://", "wss://"),
-        headers: { Authorization: authorization.presence }.compact,
+        headers: { Authorization: [authorization.presence] }.compact,
         ca_pem: certificate.presence
       }
     end

changelogs/unreleased/25990-interactive-web-terminals-authorization.yml

+---
+title: Fix authorization for interactive web terminals
+merge_request: 20811
+author:
+type: fixed

spec/models/ci/build_runner_session_spec.rb

@@ -29,7 +29,7 @@ describe Ci::BuildRunnerSession, model: true do
       it 'adds Authorization header if authorization is present' do
         subject.authorization = 'whatever'
-        expect(terminal_specification[:headers]).to include(Authorization: 'whatever')
+        expect(terminal_specification[:headers]).to include(Authorization: ['whatever'])
       end
     end
   end