Remove Ruby <2.3 base64 decoding workaround (#9)

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.

Author: rossta

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)