schema_registry_encode double encodes path params

[paulius-asvydis]

When using schema_registry_encode processor '/' character is double encoded.

With subject like:

          - schema_registry_encode:
              url: http://my-url
              subject: "main/common"
              refresh_period: 10m

The offending code seems to be:

// internal/impl/confluent/client.go

func (c *schemaRegistryClient) doRequest(ctx context.Context, verb, reqPath string) (resCode int, resBody []byte, err error) {
	reqURL := *c.schemaRegistryBaseURL
	if reqURL.Path, err = url.JoinPath(reqURL.Path, reqPath); err != nil {
		return
	}

       var req *http.Request
       // LOG ADDED FOR DEBUGGING
        fmt.Printf("reqURL: %v\n", reqURL.String())
	if req, err = http.NewRequestWithContext(ctx, verb, reqURL.String(), http.NoBody); err != nil {
		return
	}

// reqURL: http://my-url/subjects/main%252Fcommon/versions/latest

Expected Parsed URL:

http://my-url/subjects/main%2Fcommon/versions/latest

Actual Parsed URL:

http://my-url/subjects/main%252Fcommon/versions/latest

Clearly the '/' character is double encoded.

[Mizaro]

I tried it out and investigated it a bit.
It seems that the problem is in Golang's HTTP library. golang/go#3659
As far as I see, we should implement a workaround to allow any input.

I can start working on a PR to do it.