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/shared/models/image_validations.rb

@@ -6,23 +6,23 @@ shared_examples "image validations" do |imageable_factory|
   let!(:imageable)              { image.imageable }
   let!(:acceptedcontenttypes)   { imageable_accepted_content_types }
 
-  it "should be valid" do
+  it "is valid" do
     expect(image).to be_valid
   end
 
-  it "should not be valid without a title" do
+  it "is not valid without a title" do
     image.title = nil
 
     expect(image).to_not be_valid
   end
 
-  it "should not be valid without an attachment" do
+  it "is not valid without an attachment" do
     image.attachment = nil
 
     expect(image).to_not be_valid
   end
 
-  it "should be valid for all accepted content types" do
+  it "is valid for all accepted content types" do
     acceptedcontenttypes.each do |content_type|
       extension = content_type.split("/").last
       image.attachment = File.new("spec/fixtures/files/clippy.#{extension}")
@@ -31,7 +31,7 @@ shared_examples "image validations" do |imageable_factory|
     end
   end
 
-  it "should not be valid for png and gif image content types" do
+  it "is not valid for png and gif image content types" do
     ["gif", "png"].each do |content_type|
       extension = content_type.split("/").last
       image.attachment = File.new("spec/fixtures/files/clippy.#{extension}")
@@ -40,27 +40,27 @@ shared_examples "image validations" do |imageable_factory|
     end
   end
 
-  it "should not be valid for attachments larger than imageable max_file_size definition" do
+  it "is not valid for attachments larger than imageable max_file_size definition" do
     allow(image).to receive(:attachment_file_size).and_return(Image::MAX_IMAGE_SIZE + 1.byte)
 
     expect(image).to_not be_valid
     expect(image.errors[:attachment]).to include "must be in between 0 Bytes and 1 MB"
   end
 
-  it "should not be valid without a user_id" do
+  it "is not valid without a user_id" do
     image.user_id = nil
 
     expect(image).to_not be_valid
   end
 
-  it "should not be valid without a imageable_id" do
+  it "is not valid without a imageable_id" do
     image.save
     image.imageable_id = nil
 
     expect(image).to_not be_valid
   end
 
-  it "should not be valid without a imageable_type" do
+  it "is not valid without a imageable_type" do
     image.save
     image.imageable_type = nil