Expand keyring tests for sr25519 (#302)

* Expand keyring tests for sr25519

* Update index.spec.ts

* Update index.spec.ts

* Support type conversion, type per pair

* toType tests

packages/keyring/src/pair/encode.ts

@@ -2,12 +2,14 @@
 // This software may be modified and distributed under the terms
 // of the Apache-2.0 license. See the LICENSE file for details.
 
+import { PairInfo } from './types';
+
 import { stringToU8a, u8aConcat, u8aFixLength, assert } from '@polkadot/util/index';
 import { naclEncrypt } from '@polkadot/util-crypto/index';
 
 import { PKCS8_DIVIDER, PKCS8_HEADER } from './defaults';
 
-export default function encode (publicKey: Uint8Array, seed?: Uint8Array | null, passphrase?: string): Uint8Array {
+export default function encode ({ publicKey, seed }: PairInfo, passphrase?: string): Uint8Array {
   assert(seed, 'Expected a valid seed to be passed to encode');
 
   const encoded = u8aConcat(