[webui] Fix handling of stringified bolleans in index action of project controller

When a browser, or capybara, provided the (boolean) value of show_all parameter as
a string, the setting were always interpreted as true. This caused one of our tests
to fail and was also reproducable in firefox (44.0.2).

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3) Webui::ProjectController GET #index showing not home projects should eq 1

     Failure/Error: it { expect(assigns(:projects).length).to eq(1) }
       expected: 1
            got: 2

       (compared using ==)

     # ./spec/controllers/webui/project_controller_spec.rb:53:in `block (4 levels) in <top (required)>'
     # ./spec/support/logging.rb:4:in `block (2 levels) in <top (required)>'