Add save! method to ActiveModel models

This way we make it clear we expect records to be valid when we save
them, just like we do with ActiveRecord models.

spec/models/local_census_records/import_spec.rb

@@ -46,7 +46,7 @@ describe LocalCensusRecords::Import do
 
   context "#save" do
     it "Create valid local census records with provided values" do
-      import.save
+      import.save!
       local_census_record = LocalCensusRecord.find_by(document_number: "X11556678")
 
       expect(local_census_record).not_to be_nil
@@ -57,7 +57,7 @@ describe LocalCensusRecords::Import do
     end
 
     it "Add successfully created local census records to created_records array" do
-      import.save
+      import.save!
 
       valid_document_numbers = ["44556678T", "33556678T", "22556678T", "X11556678"]
       expect(import.created_records.collect(&:document_number)).to eq(valid_document_numbers)
@@ -68,7 +68,7 @@ describe LocalCensusRecords::Import do
       file = Rack::Test::UploadedFile.new(Rails.root.join(*path))
       import.file = file
 
-      import.save
+      import.save!
 
       invalid_records_document_types = [nil, "DNI", "Passport", "NIE"]
       invalid_records_document_numbers = ["44556678T", nil, "22556678T", "X11556678"]

spec/models/verification/letter_spec.rb

@@ -23,7 +23,7 @@ describe Verification::Letter do
 
     it "updates letter_requested" do
       letter = build(:verification_letter)
-      letter.save
+      letter.save!
       expect(letter.user.letter_requested_at).to be
     end
 

spec/models/verification/management/email_spec.rb

@@ -43,7 +43,7 @@ describe Verification::Management::Email do
       allow(Devise.token_generator).to receive(:generate).with(User, :email_verification_token).and_return(["1", "2"])
       allow(Mailer).to receive(:email_verification).with(user, user.email, "2", "1", "1234").and_return(mail)
 
-      validation.save
+      validation.save!
 
       expect(user.reload).to be_level_two_verified
       expect(user.document_type).to eq("1")