In order to fix the failing test in the preceding commit when optional
params are changed from their default "computed" values.
These weren't working well with `HttpHealthCheck.Patch()` because it was
attempting to set all unspecified params to Go's type defaults (eg. 0 for
int64) which the API rejected.
Changing the call to `HttpHealthCheck.Update()` seemed to fix this but it
still didn't allow you to reset a param back to it's default by no longer
specifying it.
Settings defaults like this, which match the Terraform docs, seems like the
best all round solution. Includes two additional tests for the acceptance
tests which verify the params are really getting set correctly.
Lars Wander