Make StringLineGroup returns a count limited Enumerator

Fixes #757, before we return the array enumerator directly, enumerate it will get phantom empty lines.

src/Markdig.Tests/TestStringSliceList.cs

@@ -1,3 +1,4 @@
+using System.Collections;
 using System.Text;
 
 using Markdig.Helpers;
@@ -215,4 +216,20 @@ public class TestStringSliceList
         TextAssert.AreEqual("ABC\r\nD\r\n", chars.ToString());
         TextAssert.AreEqual("ABC\r\nD", text.ToString());
     }
+
+    [Test]
+    public void TestStringLineGroup_EnumeratorReturnsRealLines()
+    {
+        string str = "A\r\n";
+        var text = new StringLineGroup(4)
+        {
+            new StringSlice(str, NewLine.CarriageReturnLineFeed) { Start = 0, End = 0 }
+        };
+
+        var enumerator = ((IEnumerable)text).GetEnumerator();
+        Assert.True(enumerator.MoveNext());
+        StringLine currentLine = (StringLine)enumerator.Current;
+        TextAssert.AreEqual("A", currentLine.ToString());
+        Assert.False(enumerator.MoveNext());
+    }
 }

src/Markdig/Helpers/StringLineGroup.cs

@@ -187,9 +187,40 @@ public struct StringLineGroup : IEnumerable
         }
     }
 
+    private struct Enumerator : IEnumerator
+    {
+        private readonly StringLineGroup _parent;
+        private int _index;
+
+        public Enumerator(StringLineGroup parent)
+        {
+            _parent = parent;
+            _index = -1;
+        }
+
+        public object Current => _parent.Lines[_index];
+
+        public bool MoveNext()
+        {
+            if (++_index < _parent.Count)
+            {
+                return true;
+            }
+            else
+            {
+                return false;
+            }
+        }
+
+        public void Reset()
+        {
+            _index = -1;
+        }
+    }
+
     IEnumerator IEnumerable.GetEnumerator()
     {
-        return Lines.GetEnumerator();
+        return new Enumerator(this);
     }
 
     private void IncreaseCapacity()