set default execution period to avoid panic

[dmarkhas]

If ExecutionPeriod option is unset on either the Health instance or the individual check, the code will panic when attempting to create a new ticker in scheduleCheck, since the execution period's default value is 0:

func NewTicker(d Duration) *Ticker {
	if d <= 0 {
		panic(errors.New("non-positive interval for NewTicker"))
	}
	....

[eranharel]

please add a test that recreates the bug

[eranharel]

I thought about this a little more, and I think that it would make more sense to return an error from Health.RegisterCheck() rather than surprise the user with a 1 min execution period.

[dmarkhas]

That would be a breaking change to the API (it will break at runtime, not at compile time) and I feel that it would make it confusing because it will start returning errors if an optional value is not provided, which kinda defeats the purpose of it being optional :-)

If a user registers a check without specifying an execution period, he probably "trusts" the library to have a safe default, and I think 1 minute accomplishes that goal.

[dmarkhas]

OTOH currently it will just panic in that condition...

Changed it to return an error + added tests

[eranharel]

LGTM, thanks Dan