Attorney’s fees — Proposal for settlement — Trial court’s determination that defendant’s proposal for settlement was invalid because certificate of service accompanying it was not signed in compliance with rule 2.515 was incorrect, as signature is not expressly required by applicable statute and rules — Even if signing requirement of rule 2.515 applied generally to rule 1.442, that requirement would nevertheless not apply to initial service of proposals for settlement on opposing party because those proposals are not “documents” as defined