Add Paper#save for saving PDFs

lib/arx/entities/paper.rb

@@ -234,6 +234,19 @@ def ==(paper)
       end
     end
 
+    # Downloads the paper and saves it in PDF format at the specified path.
+    #
+    # @param path [String] The file path to store the PDF at.
+    def save(path)
+      begin
+        pdf_content = URI.open(pdf_url).read
+        File.open(path, 'wb') {|f| f.write pdf_content}
+      rescue
+        File.delete(path) if File.file? path
+        raise
+      end
+    end
+
     inspector *ATTRIBUTES
   end
 end

spec/arx/entities/paper_spec.rb

@@ -645,4 +645,21 @@
       it { is_expected.not_to eq 1 }
     end
   end
+  context '#save' do
+    context 'with an existing directory' do
+      # TODO
+    end
+    context 'with a non-existing directory' do
+      # TODO
+    end
+    context 'with an existing file path' do
+      # TODO
+    end
+    context 'with a non-existing file path' do
+      # TODO
+    end
+    context 'with an invalid file path type' do
+      # TODO
+    end
+  end
 end