Fixed #298 sticker text issue, add text parameter.

[kkdai]

Fixed PR #298 could not parse enter text "text" from sticker directly.
Original structure Sticker doesn't have the variable.

type StickerMessage struct {
	ID                  string
	PackageID           string
	StickerID           string
	StickerResourceType StickerResourceType
	Keywords            []string

	quickReplyItems *QuickReplyItems
	sender          *Sender
	messageType     MessageType
}

[codecov-commenter]

Codecov Report

Merging #300 (571fbf6) into master (c1241cb) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #300      +/-   ##
==========================================
+ Coverage   78.50%   78.53%   +0.02%     
==========================================
  Files          37       37              
  Lines        2870     2874       +4     
==========================================
+ Hits         2253     2257       +4     
  Misses        486      486              
  Partials      131      131              

Impacted Files	Coverage Δ
linebot/event.go	97.90% <100.00%> (+0.02%)	⬆️
linebot/message.go	79.10% <100.00%> (+0.14%)	⬆️

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[kkdai]

@clsung Do you have time to help to review it?

[clsung]

LGTM