Fix for UnsupportedOperationException in user_change event

[gaurav2493]

Summary

When user_change event is sent, the code breaks with the following exception

java.lang.UnsupportedOperationException: JsonObject

	at com.google.gson.JsonElement.getAsString(JsonElement.java:179)
	at com.slack.api.bolt.middleware.builtin.IgnoringSelfEvents.apply(IgnoringSelfEvents.java:79)

This happens because code expects user to contain string but it contained JSON object

{
    "token": ".....",
    "team_id": ".....",
    "enterprise_id": "....",
    "api_app_id": "....",
    "event": {
        "type": "user_change",
        "user": {
            "id": "....",
            "name": "gaurav",
......
}

fixes #406

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #407 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #407      +/-   ##
============================================
- Coverage     83.91%   83.88%   -0.04%     
- Complexity     2367     2368       +1     
============================================
  Files           249      249              
  Lines          6368     6372       +4     
  Branches        577      578       +1     
============================================
+ Hits           5344     5345       +1     
- Misses          670      674       +4     
+ Partials        354      353       -1

Impacted Files	Coverage Δ	Complexity Δ
...pi/bolt/middleware/builtin/IgnoringSelfEvents.java	89.58% <100%> (+0.94%)	18 <0> (+1)	⬆️
...pi/methods/metrics/impl/RedisMetricsDatastore.java	87.03% <0%> (-2.47%)	37% <0%> (-1%)
...i/methods/metrics/impl/MemoryMetricsDatastore.java	92.25% <0%> (+0.7%)	42% <0%> (+1%)	⬆️

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[seratch]

This fix looks great to me. Thank you very much for your contribution!