Make TableViewHeader's delegate a weak reference

[simonbilskyrollins]

The strong reference to TableViewHeader's delegate turned out to be the source of a memory leak in my app. Changing this reference to weak allows the parent CollapsibleTableViewController to be properly deallocated.

[codecov-io]

Codecov Report

Merging #56 into master will not change coverage. The diff coverage is 0%.

@@          Coverage Diff           @@
##           master     #56   +/-   ##
======================================
  Coverage    96.5%   96.5%           
======================================
  Files           3       3           
  Lines         315     315           
======================================
  Hits          304     304           
  Misses         11      11

Impacted Files	Coverage Δ
...ionViewController/CollapsibleTableViewHeader.swift	87.93% <0%> (ø)	:arrow_up:

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