Make Address support one word names

Fixes issue solidusio#302

In the case that:
- SolidusSupport::combined_first_and_last_name_in_address returns true;
- Spree::Address::Name is not defined; and
- the given name argument to self.split_name is just one word (e.g
  "Bruce")

Then it would return the array of one element such as ['Bruce'].

lastname was assuming that there should be two elements and was calling
.last on the array, resulting in the same word as firstname. This made
the recipientName for example be "Bruce Bruce".
https://github.com/solidusio/solidus_paypal_braintree/blob/819bd3467e6f0c8ead8d855138ba88e10eb80a18/app/models/solidus_paypal_braintree/address.rb#L35

The #fullname method was added to fix the problem that if the given name
was only one word, then recipientName won't attempt to add a space
between the firstname and lastname's returned empty string. Otherwise,
it would cause for example: "Bruce ".

app/models/solidus_paypal_braintree/address.rb

@@ -32,7 +32,7 @@ def to_json(*_args)
         postalCode: zipcode,
         countryCode: country.iso,
         phone: phone,
-        recipientName: "#{firstname} #{lastname}"
+        recipientName: fullname
       }
 
       if ::Spree::Config.address_requires_state && country.states_required
@@ -41,6 +41,10 @@ def to_json(*_args)
       address_hash.to_json
     end
 
+    def fullname
+      [firstname, lastname].select(&:present?).join(' ')
+    end
+
     def firstname
       if SolidusSupport.combined_first_and_last_name_in_address?
         self.class.split_name(spree_address.name).first
@@ -51,7 +55,8 @@ def firstname
 
     def lastname
       if SolidusSupport.combined_first_and_last_name_in_address?
-        self.class.split_name(spree_address.name).last
+        split_names = self.class.split_name(spree_address.name)
+        split_names.size == 2 ? split_names.last : ''
       else
         spree_address.lastname
       end

spec/models/solidus_paypal_braintree/address_spec.rb

@@ -21,6 +21,43 @@
     end
   end
 
+  describe "#fullname" do
+    subject { described_class.new(address).fullname }
+
+    let(:address) { instance_double('address', name: name) }
+
+    context 'when Solidus combines names but Spree::Address::Name is not defined' do
+      before do
+        allow(SolidusSupport).to receive(:combined_first_and_last_name_in_address?).and_return(true)
+        allow(described_class).to receive(:defined?).with(Spree::Address::Name).and_return(false)
+      end
+
+      context 'with a one word name' do
+        let(:name) { 'Bruce' }
+
+        it 'equals "Bruce"' do
+          expect(subject).to eq('Bruce')
+        end
+      end
+
+      context 'with a one word name but extra right padding' do
+        let(:name) { 'Bruce ' }
+
+        it 'equals "Bruce"' do
+          expect(subject).to eq('Bruce')
+        end
+      end
+
+      context "with a two word name" do
+        let(:name) { 'Bruce Wayne' }
+
+        it 'equals "Bruce Wayne"' do
+          expect(subject).to eq('Bruce Wayne')
+        end
+      end
+    end
+  end
+
   describe '#to_json' do
     subject(:address_json) { JSON.parse(described_class.new(spree_address).to_json) }