Fix checking http response code

The status code is a string, not a number.

From ruby docs:

> The HTTP result code string. For example, ‘302’. You can also
determine the response type by examining which response subclass the
response object is an instance of.

lib/zipkin/json_client.rb

@@ -39,7 +39,7 @@ def emit_batch(spans)
       request.body = JSON.dump(spans)
       response = http.request(request)
 
-      if response.code != 202
+      if response.code != '202'
         @logger.error("Received bad response from Zipkin. status: #{response.code}, body: #{response.body.inspect}")
       end
     rescue StandardError => e