Add assertions to mark, unmark, deadline

[codecov[bot]]

Codecov Report

Merging #232 (57245a8) into master (39b5428) will decrease coverage by 0.32%. The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #232      +/-   ##
============================================
- Coverage     70.92%   70.60%   -0.32%     
- Complexity      568      571       +3     
============================================
  Files           101      101              
  Lines          1909     1919      +10     
  Branches        166      171       +5     
============================================
+ Hits           1354     1355       +1     
  Misses          501      501              
- Partials         54       63       +9     

Files	Coverage Δ
...in/java/seedu/address/logic/parser/ParserUtil.java	96.92% <ø> (ø)
.../java/seedu/address/logic/commands/AddCommand.java	86.88% <0.00%> (-2.95%)	:arrow_down:
...c/main/java/seedu/address/model/task/Deadline.java	70.00% <0.00%> (-5.00%)	:arrow_down:
...java/seedu/address/logic/commands/MarkCommand.java	88.88% <0.00%> (-11.12%)	:arrow_down:
...va/seedu/address/logic/commands/UnmarkCommand.java	88.88% <0.00%> (-11.12%)	:arrow_down:

... and 1 file with indirect coverage changes