Fix regression for html block parsing (issue #21)

src/Markdig.Tests/TestPlayParser.cs

@@ -51,6 +51,23 @@ Later in a text we are using HTML and it becomes an abbr tag HTML
         }
 
 
+        [Test]
+        public void TestHtmlBug()
+        {
+            TestParser.TestSpec(@" # header1
+
+<pre class='copy'>
+blabla
+</pre>
+
+# header2
+", @"<h1>header1</h1>
+<pre class='copy'>
+blabla
+</pre>
+<h1>header2</h1>");
+        }
+
 
 
         [Test]

src/Markdig/Helpers/StringSlice.cs

@@ -249,7 +249,7 @@ namespace Markdig.Helpers
             endOfIndex = 0;
             for (int i = Start; i <= end; i++)
             {
-                if (MatchLowercase(text, End, i))
+                if (MatchLowercase(text, End, i - Start))
                 {
                     endOfIndex = i + text.Length;
                     return true;