Add an integration test for --header option

[jcamiel]

Add an integration test for --header with this file:

integration/hurl/tests_ok/add_header.hurl

GET http://localhost:8000/add-header-0
HTTP 200

GET http://localhost:8000/add-header-1
header-a: foo
HTTP 200

GET http://localhost:8000/add-header-2
header-b: bar
{"message":"hi!"}
HTTP 200

and the script file integration/hurl/tests_ok/add_header.sh

$ hurl --header 'header-b:baz" --header 'header-c:qux' add_header.hurl

The Flask file integration/hurl/tests_ok/add_header.py should have 3 endpoint add_header_0, add_header_1, add_header_2 that test request headers do contain the right headers.

[theoforger]

@jcamiel I'll take this one. Might as well😆