Fix bug where connection would be closed without returning unhandled response for unknown messages (#747)

Sources/Packages/Sources/SecretAgentKit/Agent.swift

@@ -47,6 +47,7 @@ extension Agent {
                 logger.debug("Agent returned \(SSHAgent.Response.agentSignResponse.debugDescription)")
             case .unknown(let value):
                 logger.error("Agent received unknown request of type \(value).")
+                throw UnhandledRequestError()
             default:
                 logger.debug("Agent received valid request of type \(request.debugDescription), but not currently supported.")
                 throw UnhandledRequestError()