Change the email value, '', into "#{SecureRandom.uuid}@example.com"

- `"#{provider}-#{uid}@example.com"` breaks backward compatibility because
in wiki it says `create_from_provider(provider)`: it doesn't pass uid.

- `@example.com` makes it easy to check which email is valid or not

lib/sorcery/model/submodules/external.rb

@@ -61,7 +61,7 @@ def create_from_provider(provider, uid, attrs)
 
             # For external services that don't offer an email like twitter.
             unless attrs.has_key?(:email) && user.attributes.has_key?('email')
-              user.send(:'email=', '')
+              user.send(:'email=', "#{SecureRandom.uuid}@example.com")
             end
 
             if block_given?

spec/shared_examples/user_shared_examples.rb

@@ -551,7 +551,7 @@ def self.matches?(crypted, *tokens)
           User.create_from_provider('facebook', '123', username: 'Noam Ben Ari') { true }
         end.to change { User.count }.by(1)
 
-        expect(User.first.email).to eq ''
+        expect(User.first.email).not_to be_empty
       end
     end