Implements equals and hash code overrides for completion proposal

[jphughes88]

Added with @georgiacarr1999

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[codecov-io]

Codecov Report

Merging #244 into master will decrease coverage by 0.02%.
The diff coverage is 22.22%.

@@             Coverage Diff              @@
##             master     #244      +/-   ##
============================================
- Coverage     48.91%   48.88%   -0.03%     
- Complexity      252      255       +3     
============================================
  Files            53       53              
  Lines          1333     1342       +9     
  Branches        204      210       +6     
============================================
+ Hits            652      656       +4     
+ Misses          625      623       -2     
- Partials         56       63       +7

Impacted Files	Coverage Δ	Complexity Δ
.../org/springframework/shell/CompletionProposal.java	51.72% <22.22%> (-3.28%)	8 <2> (+3)

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[stephen-murby]

@ericbottard Wonder if you could help with this PR, wondering about the coverage. Is it usual for you to write tests against POJO's or should the class be added to an exclusion list?

If you are not the right person, firstly, apologies, secondly are you able to point us in the right direction? I wasn't able to find any contributing docs with an IRC or slack channel etc...