Added configuration content

Necessary to configure the client.

lib/trmnl/api/configuration/content.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+module TRMNL
+  module API
+    module Configuration
+      # Provides API client configuration content.
+      Content = Struct.new :content_type, :uri do
+        def headers = {"Content-Type" => content_type}.compact
+      end
+    end
+  end
+end

spec/lib/trmnl/api/configuration/content_spec.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe TRMNL::API::Configuration::Content do
+  subject :content do
+    described_class[uri: "https://trmnl.app/api", content_type: "application/json"]
+  end
+
+  describe "#headers" do
+    it "answers HTTP headers when defined" do
+      expect(content.headers).to eq("Content-Type" => "application/json")
+    end
+
+    it "answers empty hash when none are available" do
+      content.content_type = nil
+      expect(content.headers).to eq({})
+    end
+  end
+end