Remove Ruby <2.3 base64 decoding workaround

In older Ruby versions, WebPush.decode64 was introduced to wrap
Base64.urlsafe_decode64 to prevent errors decoding "unpadded"
urlsafe-encoded input. Since Ruby 2.3, Base64.urlsafe_decode64 can now
gracefully decode padded and unpadded input. Given WebPush now supports
Ruby 3+, we should be able to remove this workaround gracefully. I've
added basic specs to demonstrate and perhaps catch any potential (though
unlikely) future regressions in Ruby.

For historical discussion of the issue:
https://bugs.ruby-lang.org/issues/10740

lib/web_push.rb

@@ -56,10 +56,6 @@ def encode64(bytes)
     end
 
     def decode64(str)
-      # For Ruby < 2.3, Base64.urlsafe_decode64 strict decodes and will raise errors if encoded value is not properly padded
-      # Implementation: http://ruby-doc.org/stdlib-2.3.0/libdoc/base64/rdoc/Base64.html#method-i-urlsafe_decode64
-      str = str.ljust((str.length + 3) & ~3, '=') if !str.end_with?('=') && str.length % 4 != 0
-
       Base64.urlsafe_decode64(str)
     end
 

spec/web_push_spec.rb

@@ -5,6 +5,18 @@
     expect(WebPush::VERSION).not_to be nil
   end
 
+  describe '.decode64' do
+    let(:a_padded_key) { "YWJjZGU=" }
+
+    it 'urlsafe decodes padded base64 string' do
+      expect(WebPush.decode64(a_padded_key)).to eq("abcde")
+    end
+
+    it 'urlsafe decodes unpadded base64 string' do
+      expect(WebPush.decode64(a_padded_key.delete("="))).to eq("abcde")
+    end
+  end
+
   shared_examples 'web push protocol standard error handling' do
     it 'raises InvalidSubscription if the API returns a 404 Error' do
       stub_request(:post, expected_endpoint)