Correction to the threshold_windows

[heldersepu]

This fixes issue #259
The threshold_windows should be added only if a window has been set

The Options structure already has the flag to omit it when it is empty:

type Options struct {
        ...
	EscalationMessage *string           `json:"escalation_message,omitempty"`
	Thresholds        *ThresholdCount   `json:"thresholds,omitempty"`
	ThresholdWindows  *ThresholdWindows `json:"threshold_windows,omitempty"`
	IncludeTags       *bool             `json:"include_tags,omitempty"`
        ...
}

[bkabrda]

Looks perfect 👍 Thank you very much for the contribution! Merging.

[heldersepu]

Wow! Thanks, @bkabrda that was quick!