Implement NIP-19 bech32-encoded private and public keys

https://github.com/nostr-protocol/nips/blob/master/19.md

spec/nostr/key_pair_spec.rb

@@ -5,19 +5,43 @@ require 'spec_helper'
 RSpec.describe Nostr::KeyPair do
   let(:keypair) do
     described_class.new(
-      private_key: '893c4cc8088924796b41dc788f7e2f746734497010b1a9f005c1faad7074b900',
-      public_key: '2d7661527d573cc8e84f665fa971dd969ba51e2526df00c149ff8e40a58f9558'
+      private_key: Nostr::PrivateKey.new('893c4cc8088924796b41dc788f7e2f746734497010b1a9f005c1faad7074b900'),
+      public_key: Nostr::PublicKey.new('2d7661527d573cc8e84f665fa971dd969ba51e2526df00c149ff8e40a58f9558')
     )
   end
 
   describe '.new' do
-    it 'creates an instance of a key pair' do
-      keypair = described_class.new(
-        private_key: '893c4cc8088924796b41dc788f7e2f746734497010b1a9f005c1faad7074b900',
-        public_key: '2d7661527d573cc8e84f665fa971dd969ba51e2526df00c149ff8e40a58f9558'
-      )
+    context 'when private_key is not an instance of PrivateKey' do
+      it 'raises an error' do
+        expect do
+          described_class.new(
+            private_key: '893c4cc8088924796b41dc788f7e2f746734497010b1a9f005c1faad7074b900',
+            public_key: Nostr::PublicKey.new('2d7661527d573cc8e84f665fa971dd969ba51e2526df00c149ff8e40a58f9558')
+          )
+        end.to raise_error(ArgumentError, 'private_key is not an instance of PrivateKey')
+      end
+    end
 
-      expect(keypair).to be_an_instance_of(described_class)
+    context 'when public_key is not an instance of PublicKey' do
+      it 'raises an error' do
+        expect do
+          described_class.new(
+            private_key: Nostr::PrivateKey.new('893c4cc8088924796b41dc788f7e2f746734497010b1a9f005c1faad7074b900'),
+            public_key: '2d7661527d573cc8e84f665fa971dd969ba51e2526df00c149ff8e40a58f9558'
+          )
+        end.to raise_error(ArgumentError, 'public_key is not an instance of PublicKey')
+      end
+    end
+
+    context 'when private_key is an instance of PrivateKey and public_key is an instance of PublicKey' do
+      it 'creates an instance of a key pair' do
+        keypair = described_class.new(
+          private_key: Nostr::PrivateKey.new('893c4cc8088924796b41dc788f7e2f746734497010b1a9f005c1faad7074b900'),
+          public_key: Nostr::PublicKey.new('2d7661527d573cc8e84f665fa971dd969ba51e2526df00c149ff8e40a58f9558')
+        )
+
+        expect(keypair).to be_an_instance_of(described_class)
+      end
     end
   end