Prioritise user_id over email in user tagging (Fixes: #197)

[thewheat]

Fix for #197

• Currently library fails tagging users via Tag.tag(tag, user);
  • if the user object has both user_id and email
  • and the email is used on multiple records
• Due to code below where we prioritise email over user_id
  

  intercom-java/intercom-java/src/main/java/io/intercom/api/Tag.java

  Lines 83 to 94 in f50c869

  	if (!Strings.isNullOrEmpty(id)) {
  	userMap.put("id", id);
  	usersLite.add(userMap);
  	} else if (!Strings.isNullOrEmpty(email)) {
  	userMap.put("email", email);
  	usersLite.add(userMap);
  	} else if (!Strings.isNullOrEmpty(userId)) {
  	userMap.put("user_id", userId);
  	usersLite.add(userMap);
  	} else {
  	logger.warn("no identifiers found for user tag target, skipping [" + tag + "] [" + user.toString() + "]");
  	}

Current library would fail on this

        User user = new User().setUserId(USER_ID).setEmail(EMAIL_THAT_IS_ON_MULTIPLE_RECORDS);
        Tag tag = new Tag().setName("test-tag");
        tag = Tag.create(tag);
        Tag.tag(tag, user);

Essentially the code above would create this API request

{
  "name": "test-tag",
  "users":[
    {
      "email" : EMAIL_THAT_IS_ON_MULTIPLE_RECORDS
    }
   ]
}

Instead of

{
  "name": "test-tag",
  "users":[
    {
      "user_id" : USER_ID
    }
   ]
}

Reshuffling so user_id has precendence over this it will fix it