Enables RSpec/ExampleWording and fixes all issues

Both avoiding 'should' and repiting 'it' on the tests description
improves reading them and also makes all descriptions consistent.

Read about cop at http://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/ExampleWording

spec/models/poll/booth_assignment_spec.rb

@@ -5,7 +5,7 @@ describe Poll::BoothAssignment do
   let(:booth){create(:poll_booth)}
   let(:booth1){create(:poll_booth)}
 
-  it "should check if there are shifts" do
+  it "checks if there are shifts" do
     assignment_with_shifts = create(:poll_booth_assignment, poll: poll, booth: booth)
     assignment_without_shifts = create(:poll_booth_assignment, poll: poll, booth: booth1)
     officer = create(:poll_officer)
@@ -16,7 +16,7 @@ describe Poll::BoothAssignment do
     expect(assignment_without_shifts.shifts?).to eq(false)
   end
 
-  it "should delete shifts associated to booth assignments" do
+  it "deletes shifts associated to booth assignments" do
     assignment = create(:poll_booth_assignment, poll: poll, booth: booth)
     officer = create(:poll_officer)
     create(:poll_officer_assignment, officer: officer, booth_assignment: assignment)