Fix error when saving braintree payment method

If you try to update the Braintree payment method in the admin it will
throw an error due to 2 of the preference types being hashes. Solidus
automatically creates a text field for these on the page, which it
populates with the default value: "{}". The code inside
`Spree::Preferences::Preferable` that handles type conversion throws an
error if the input for a preference with type :hash is not a Hash
object.

This change allows for a well-formed hash to be input as a string and
converts it to a Hash before saving the record.

app/models/solidus_paypal_braintree/gateway.rb

@@ -204,6 +204,17 @@ def cents(dollars)
       dollars.to_money.cents
     end
 
+    def to_hash(preference_string)
+      JSON.parse(preference_string.gsub("=>", ":"))
+    end
+
+    def convert_preference_value(value, type)
+      if type == :hash && value.is_a?(String)
+        value = to_hash(value)
+      end
+      super
+    end
+
     def transaction_options(source, options, submit_for_settlement = false)
       params = options.select do |key, _|
         ALLOWED_BRAINTREE_OPTIONS.include?(key)

spec/models/solidus_paypal_braintree/gateway_spec.rb

@@ -20,6 +20,35 @@
   end
   let(:payment_type) { SolidusPaypalBraintree::Source::PAYPAL }
 
+  describe "saving preference hashes as strings" do
+    subject { gateway.update(update_params) }
+
+    context "with valid hash syntax" do
+      let(:update_params) do
+        {
+          preferred_merchant_currency_map: '{"EUR" => "test_merchant_account_id"}',
+          preferred_paypal_payee_email_map: '{"CAD" => "[email protected]"}'
+        }
+      end
+
+      it "successfully updates the preference" do
+        subject
+        expect(gateway.preferred_merchant_currency_map).to eq({ "EUR" => "test_merchant_account_id" })
+        expect(gateway.preferred_paypal_payee_email_map).to eq({ "CAD" => "[email protected]" })
+      end
+    end
+
+    context "with invalid user input" do
+      let(:update_params) do
+        { preferred_merchant_currency_map: '{this_is_not_a_valid_hash}' }
+      end
+
+      it "raise a JSON parser error" do
+        expect{ subject }.to raise_error(JSON::ParserError)
+      end
+    end
+  end
+
   describe 'making a payment on an order', vcr: { cassette_name: 'gateway/complete' } do
     include_context 'order ready for payment'