Add email to Braintree customer

Issue: solidusio#309

Braintree comes with free basic fraud protection. One of the tools
is Risk Threshold Rules (velocity checks) [1]. One of the checks is
checking amount of transactions per customer email. However, the email
was not being attached to the customer when creating it before the
transaction.

This fixes that so developers can utilize this check.

[1] https://developer.paypal.com/braintree/articles/guides/fraud-tools/basic/risk-threshold-rules

app/models/solidus_paypal_braintree/gateway.rb

@@ -398,6 +398,8 @@ def paypal_payee_email_for(source, options)
     def customer_profile_params(payment)
       params = {}
 
+      params[:email] = payment&.order&.email
+
       if store_in_vault && payment.source.try(:nonce)
         params[:payment_method_nonce] = payment.source.nonce
       end

spec/models/solidus_paypal_braintree/gateway_spec.rb

@@ -544,6 +544,56 @@
       end
     end
 
+    describe '#customer_profile_params' do
+      subject(:params) { gateway.send(:customer_profile_params, payment) }
+
+      let(:payment) do
+        build(:payment, {
+          payment_method: gateway,
+          source: source
+        })
+      end
+
+      context 'when payment does not belong to an order' do
+        before { allow(payment).to receive(:order).and_return(nil) }
+
+        it 'has the email param as nil' do
+          expect(subject[:email]).to be_nil
+        end
+      end
+
+      context 'when payment belongs to an order' do
+        it 'has no email param' do
+          expect(subject[:email]).to eq(payment.order.email)
+        end
+      end
+    end
+
+    describe "Braintree Customer" do
+      subject(:customer) { braintree.customer.create(params).customer }
+
+      let(:params) { gateway.send(:customer_profile_params, payment) }
+
+      let(:payment) do
+        build(:payment, {
+          payment_method: gateway,
+          source: source
+        })
+      end
+
+      cassette_options = {
+        cassette_name: 'gateway/customer',
+        match_requests_on: [:braintree_uri]
+      }
+
+      context "with customer", vcr: cassette_options do
+        it 'saves the customer email correctly' do
+          allow(payment.order).to receive(:email).and_return('[email protected]')
+          expect(subject.email).to eq(payment.order.email)
+        end
+      end
+    end
+
     shared_examples "sources_by_order" do
       let(:order) { FactoryBot.create :order, user: user, state: "complete", completed_at: Time.current }
       let(:gateway) { new_gateway.tap(&:save!) }