title: Issue while including enum type in unions within avro schema
body: <p>I am working with Apache Kafka to send messages to Kafka topics. I am trying to use unions in Avro Schemas including enum types for message validation. But I am facing an issue with the usage of enum types within union. I am using Kafka REST API through POSTMAN tool to post a record/message to a topic with schema validation. Below is the request payload including schema and records inline - </p>
  
  <pre><code>{
      "key_schema": "{\"type\": \"record\", \"name\": \"key\", \"fields\": [{\"name\": \"keyInput\", \"type\": \"string\"}]}",
      "value_schema": "{\"type\": \"record\", \"name\": \"value\", \"fields\": [{\"name\": \"valueInput1\", \"type\": \"string\"},{\"name\": \"valueInput2\",\"type\":[{\"type\":\"enum\",\"name\":\"actorobjType\",\"symbols\":[\"Agent\",\"Group\"]},\"null\"],\"default\":null}]}",
      "records": [
          {
              "key": {
                  "keyInput": "testUser-key"
              },
              "value": {
                  "valueInput1": "testUser-value",
                  "valueInput2": "Agent"
              }
          }
      ]
  }
  </code></pre>
  
  <p>I am getting the following error when I am trying to insert a record using above request payload -</p>
  
  <pre><code>{
    "error_code": 42203,
    "message": "Conversion of JSON to Avro failed: Failed to convert JSON to Avro: Expected start-union. Got VALUE_STRING"
  }
  </code></pre>
  
  <p>After searching in different sites including stackoverflow, I came through a <a href="https://stackoverflow.com/questions/27485580/how-to-fix-expected-start-union-got-value-number-int-when-converting-json-to-av">suggestion</a></p>
  
  <p>asking to explicitly specify the type while passing the record as below - </p>
  
  <pre><code>{
      "key_schema": "{\"type\": \"record\", \"name\": \"key\", \"fields\": [{\"name\": \"keyInput\", \"type\": \"string\"}]}",
      "value_schema": "{\"type\": \"record\", \"name\": \"value\", \"fields\": [{\"name\": \"valueInput1\", \"type\": \"string\"},{\"name\": \"valueInput2\",\"type\":[{\"type\":\"enum\",\"name\":\"actorobjType\",\"symbols\":[\"Agent\",\"Group\"]},\"null\"],\"default\":null}]}",
      "records": [
          {
              "key": {
                  "keyInput": "testUser-key"
              },
              "value": {
                  "valueInput1": "testUser-value",
                  "valueInput2": {
                      "enum": "Agent"
                  }
              }
          }
      ]
  }
  </code></pre>
  
  <p>But then I face the below error -</p>
  
  <pre><code>{
    "error_code": 42203,
    "message": "Conversion of JSON to Avro failed: Failed to convert JSON to Avro: Unknown union branch enum"
  }
  </code></pre>
  
  <p>The same suggestion worked fine for unions with other types like string and map, but with unions including enum, that does not seem to work.</p>
  
  <p>I also thought there may be some other type which needs to be used for enum specification, Hence I tried some other words like below -</p>
  
  <pre><code>"valueInput2": {
                      "string": "Agent"
                  }
  </code></pre>
  
  <p>and</p>
  
  <pre><code>"valueInput2": {
                      "enumeration": "Agent"
                  }
  </code></pre>
  
  <p>But none of them seem to work. Please help me resolve this.</p>