Add a validation for "start_at" and "finish_at"
Implement "Schedule#collide_to?"
Fix a fixture
Add test cases
Fix Schedule#collide_to?
Add test cases
Add a test case
Implement "Schedule#span_not_collide_to_others?"
Add a validation for span collision
Fix from "create" to "build"
Test the validation for collision to others
Merge branch 'collide-to-others' into span-test
Merge branch 'span-test'