MT#6295 API: allow tests without invalid/unauth cert

which are (for now) difficult to get on test machines

(cherry picked from commit 6acd3f210d0aba0e5e1b22bb49b9cd3771a05a3b)
MT#6295 API: fix tests Prefer header no longer required

(cherry picked from commit edd904ce45f1f1da187171b0da7b956e067b4c1a)

confilcts have been resolved

Conflicts:

	t/api-billingfees.t
	t/api-billingprofiles.t
	t/api-contracts.t
	t/api-customercontacts.t
	t/api-customers.t
	t/api-resellers.t
	t/api-rewriterulesets.t
	t/api-systemcontacts.t
	t/api-valid-patch.t
MT#6295 fix api-root test

Update currently available links

(cherry picked from commit e4d2a22ef623d7c672db15a68fcda7be1dab62dc)
MT#6295 API: fix failing customers and contracts requests

validate max_subscribers (like in earlier versions) but don't
require product to be set
and typo

(cherry picked from commit 6d17b70ab0ceb8e7e6f1e29ae7582c4af8290f1a)