Make Snapshot#crop return the snapshot of exact requested height

app/models/snapshot.rb

@@ -18,8 +18,13 @@ class Snapshot
   end
 
   def crop(width, height)
-    height = [height, lines.size].min
-    new_lines = lines.drop(lines.size - height).map { |line| line.crop(width) }
+    min_height = [height, lines.size].min
+    new_lines = lines.drop(lines.size - min_height).map { |line| line.crop(width) }
+
+    while new_lines.size < height
+      new_lines << []
+    end
+
     self.class.new(new_lines)
   end
 

spec/models/snapshot_spec.rb

@@ -98,9 +98,9 @@ describe Snapshot do
     context "when height is greater than lines count" do
       let(:height) { 4 }
 
-      it 'returns a new Snapshot with all lines cropped' do
+      it 'returns a new Snapshot with all lines cropped and missing lines added' do
         expect(subject).to eq(Snapshot.new([cropped_line_1, cropped_line_2,
-                                            cropped_line_3]))
+                                            cropped_line_3, []]))
       end
     end
   end