Bug fix: DoneTaskCommand undo/redo

[koh-jx]

Bugfix for not being able to redo done task.

Issue lied in the fact that this.execute() in undo() calls super.canExecute(), which negates its previous super.canUndo().
Changed it such that it does not call this.execute().

[codecov-commenter]

Codecov Report

Merging #133 (e33e246) into master (7f739b6) will increase coverage by 0.30%.
The diff coverage is 61.53%.

@@             Coverage Diff              @@
##             master     #133      +/-   ##
============================================
+ Coverage     58.41%   58.72%   +0.30%     
- Complexity      599      602       +3     
============================================
  Files           121      121              
  Lines          2561     2563       +2     
  Branches        251      251              
============================================
+ Hits           1496     1505       +9     
+ Misses          975      966       -9     
- Partials         90       92       +2     

Impacted Files	Coverage Δ
...u/address/logic/commands/task/DoneTaskCommand.java	78.57% <61.53%> (+1.64%)	⬆️
...in/java/seedu/address/commons/util/StringUtil.java	89.28% <0.00%> (+25.00%)	⬆️

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[jeffsieu]

I would suggest extracting everything except super.canExecute() from the execute method into a private method, so that you don't repeat this code in both execute and undo.

[jeffsieu]

LGTM. I'll make some changes in a subsequent PR.