[Rails5] Fix spec/controllers/projects/jobs_controller_spec.rb
[Rails5] Fix spec/controllers/projects/jobs_controller_spec.rb

In Rails 5.0 `response.content_type` does not return charset which is expected in specs.
This commit replaces `response.content_type` with `response.headers["Content-Type"]` in specs.

mmkassem/gitlabhq,stoplightio/gitlabhq,dreampet/gitlab,dreampet/gitlab,axilleas/gitlabhq,dreampet/gitlab,jirutka/gitlabhq,axilleas/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,stoplightio/gitlabhq,axilleas/gitlabhq,mmkassem/gitlabhq,jirutka/gitlabhq,dreampet/gitlab,jirutka/gitlabhq,axilleas/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,iiet/iiet-git