Override previous patterns on new sampler

[OiCMudkips]

This addresses #61 by allowing users to check a box to override the response/disconnect regex's specified in the first WebSocket sampler.

When a user checks the checkbox, the response/close pattern on that row overwrites the previous response/close pattern associated with that connection, even if the current sampler's regex is blank (this would clear the previous pattern).

[liliasapurina]

Thank you very much for that changes! It fix my problems.