Fix tests after #846
Fix tests after #846

Change checkResponse to only expect the configured
X-Content-Type-Options header if it doesn't receive a 405 error, which
means the handler isn't registered for that method.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

thaJeztah/distribution,thaJeztah/distribution