Boolean fields are not deserialized into SlackUser model

[omotnyk]

Describe the bug
We noticed that Optional<Boolean> values are not deserialized correctly into SlackUser. For example, Slack sends user with key is_bot but we expect bot due to the Jackson deserialization logic which works like that for isBot methods. We need to add get prefix to those methods to fix deserialization or specify key via @JsonProperty. The second option seems better as there are already some usages of the methods.
#180

To Reproduce
Steps to reproduce the behavior:

1. Try to deserialize SlackUser.
2. See that no fields with isXxx getter are deserialized.

Expected behavior
All the fields with isXxx getter are deserialized correctly.