Add specs for Rex::ImageSource::Memory#initialize

spec/lib/rex/image_source/memory_spec.rb

@@ -5,10 +5,32 @@ require 'rex/image_source/memory'
 
 describe Rex::ImageSource::Memory do
 
+  let(:raw_data) { "ABCDEFGHIJKLMNOP"}
+
+  subject do
+    described_class.new(raw_data)
+  end
+
   describe "#initialize" do
     subject(:memory_class) do
       described_class.allocate
     end
+
+    it "initializes size to data length" do
+      memory_class.send(:initialize, raw_data)
+      expect(memory_class.size).to eq(raw_data.length)
+    end
+
+    it "initializes file_offset to 0 by default" do
+      memory_class.send(:initialize, raw_data)
+      expect(memory_class.file_offset).to eq(0)
+    end
+
+    context "when using nil as data" do
+      it "raises an error" do
+        expect { memory_class.send(:initialize, nil) }.to raise_error(NoMethodError)
+      end
+    end
   end
 
   describe "#read" do