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/document_validations.rb

@@ -7,23 +7,23 @@ shared_examples "document validations" do |documentable_factory|
   let!(:maxfilesize)            { max_file_size(document.documentable.class) }
   let!(:acceptedcontenttypes)   { accepted_content_types(document.documentable.class) }
 
-  it "should be valid" do
+  it "is valid" do
     expect(document).to be_valid
   end
 
-  it "should not be valid without a title" do
+  it "is not valid without a title" do
     document.title = nil
 
     expect(document).to_not be_valid
   end
 
-  it "should not be valid without an attachment" do
+  it "is not valid without an attachment" do
     document.attachment = nil
 
     expect(document).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
       document.attachment = File.new("spec/fixtures/files/empty.#{extension}")
@@ -32,27 +32,27 @@ shared_examples "document validations" do |documentable_factory|
     end
   end
 
-  it "should not be valid for attachments larger than documentable max_file_size definition" do
+  it "is not valid for attachments larger than documentable max_file_size definition" do
     document.stub(:attachment_file_size).and_return(maxfilesize.megabytes + 1.byte)
 
     expect(document).to_not be_valid
     expect(document.errors[:attachment]).to include "must be in between 0 Bytes and #{maxfilesize} MB"
   end
 
-  it "should not be valid without a user_id" do
+  it "is not valid without a user_id" do
     document.user_id = nil
 
     expect(document).to_not be_valid
   end
 
-  it "should not be valid without a documentable_id" do
+  it "is not valid without a documentable_id" do
     document.save
     document.documentable_id = nil
 
     expect(document).to_not be_valid
   end
 
-  it "should not be valid without a documentable_type" do
+  it "is not valid without a documentable_type" do
     document.save
     document.documentable_type = nil