Allow headers to be replaced on the HTTP client (#220)
* ⚙️chore(gitignore) add '*.bak' to ignore list
* ⚙️chore(dependencies) add symfony/http-foundation dependency
* ⚙️ chore(http-client) add replaceHeaders method for PendingRequest class
* Refactor TestCase
* ⚙️chore(tests) Add test for replacing headers when no headers yet set