Added targetPool:AggregatedList, getHealth. Updated HttpHealthCheck

This adds coverage of TargetPoolApi:AggregatedList and TargetPoolApi:GetHealth.

Also updates HttpHealthChecks. Defaults were not being set in HttpHealthChecks. This was hidden by the fact that we assumed default values when nothing was returned in the request. This PR addresses this and changes HttpHealthCheckCreationOptions to enable making a request with or without the default values.

Also converts TargetPoolApi to mock test.

Turns out a force push to squash a comment edit made it so I could not reopen PR #105 
I think you are correct. This means that add/remove healthCheck and add/remove instance are not nullable either.