SimpleInspect#inspect(true) raises ArgumentError #191

Docx::SimpleInspect#inspect(full = false) is documented to fall back to the default inspect when called as inspect(true). But it uses a bare super, which forwards the argument to Object#inspect, and that method takes none.

Docx::Document.open('spec/fixtures/basic.docx').inspect(true)
# => ArgumentError: wrong number of arguments (given 1, expected 0)

Fix: call super() instead of super in lib/docx/helpers.rb.

Found while splitting the specs into per-class unit specs (#188).