Fix #415 attachments payload's validity issue

[seratch]

Summary

This pull request fixes #415 where posting a message with attachments gets an error response from the server-side saying the attachments are invalid.

{
  "ok": false,
  "error": "invalid_attachments",
  "response_metadata": {
    "messages": [
      "[ERROR] invalid_keys"
    ]
  }
}

The reason of the error is that an attachment payload this library generates contains empty arrays for fields, mrkdwnIn, and actions by default. This pull request removes those default values from an attachment initialization.

Requirements (place an x in each [ ])

• 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 #416 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #416      +/-   ##
============================================
- Coverage     83.92%   83.86%   -0.07%     
+ Complexity     2368     2367       -1     
============================================
  Files           249      249              
  Lines          6372     6372              
  Branches        578      578              
============================================
- Hits           5348     5344       -4     
- Misses          670      674       +4     
  Partials        354      354              

Impacted Files	Coverage Δ	Complexity Δ
.../src/main/java/com/slack/api/model/Attachment.java	100.00% <ø> (ø)	10.00 <0.00> (ø)
...pi/methods/metrics/impl/RedisMetricsDatastore.java	87.03% <0.00%> (-2.47%)	37.00% <0.00%> (-1.00%)

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