Fix .default_taxjar_client

In #34, we modified `.default_taxjar_client` to include the API version,
and then the name of the plugin, but we also introduced a critical bug.
`Taxjar::Client#set_api_config` returns a hash and not the TaxJar
client, so it is not safe to chain the method call. As such, we add a
spec to cover this case and patch the class method.

Co-authored-by: Noah Silvera <[email protected]>

lib/super_good/solidus_taxjar/api.rb

@@ -2,13 +2,15 @@ module SuperGood
   module SolidusTaxjar
     class Api
       def self.default_taxjar_client
-        ::Taxjar::Client.new(
+        client = ::Taxjar::Client.new(
           api_key: ENV.fetch("TAXJAR_API_KEY"),
           api_url: ENV.fetch("TAXJAR_API_URL") { "https://api.taxjar.com" } # Sandbox URL: https://api.sandbox.taxjar.com
-        ).set_api_config('headers', {
+        )
+        client.set_api_config('headers', {
           'x-api-version' => '2020-08-07',
           'plugin' => 'supergoodsolidustaxjar'
         })
+        client
       end
 
       def initialize(taxjar_client: self.class.default_taxjar_client)

spec/super_good/solidus_taxjar/api_spec.rb

@@ -17,6 +17,19 @@
     end
   end
 
+  describe ".default_taxjar_client" do
+    subject { described_class.default_taxjar_client }
+
+    before do
+      ENV["TAXJAR_API_KEY"] = 'taxjar_api_token'
+    end
+
+    it "returns an instance of the TaxJar client" do
+      expect(subject).to be_an_instance_of(::Taxjar::Client)
+    end
+  end
+
+
   describe "#tax_for" do
     subject { api.tax_for order }