fix: fromXml for self-closing child twiml

[bcanseco]

Fixes

This is autogenerated output from internal PR: https://code.hq.twilio.com/twilio/yoyodyne/pull/567

XML deserialization for TwiML (added in #661) has a bug where self-closing tags can't be deserialized within parent tags.

• Deserialization works:

  SsmlBreak.Builder.fromXml("<break/>");

• Deserialization doesn't work:

  Say.Builder.fromXml("<Say><break/></Say>");

  com.twilio.twiml.TwiMLException: Failed to deserialize a Say.Builder from the provided XML string: Problem deserializing property 'break' (expected type: [simple type, class com.twilio.twiml.voice.SsmlBreak]; actual type: `com.twilio.twiml.voice.SsmlBreak$Builder`), problem: argument type mismatch at [Source: (StringReader); line: 1, column: 6] (through reference chain: com.twilio.twiml.voice.Say$Builder["break"])

The tests added in this PR fail for these TwiML elements, but then pass with the annotation I also added.

Checklist

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• [x] I have added inline documentation to the code I modified (N/A)