Fix issue while detecting generic attributes that was breaking parsing (issue #16)

src/Markdig/Extensions/GenericAttributes/GenericAttributesExtension.cs

@@ -46,7 +46,7 @@ namespace Markdig.Extensions.GenericAttributes
             // Try to find if there is any attributes { in the info string on the first line of a FencedCodeBlock
             if (line.Start < line.End)
             {
-                var indexOfAttributes = line.Text.LastIndexOf('{', line.End);
+                int indexOfAttributes = line.IndexOf('{');
                 if (indexOfAttributes >= 0)
                 {
                     // Work on a copy

src/Markdig/Helpers/StringSlice.cs

@@ -221,6 +221,22 @@ namespace Markdig.Helpers
             return false;
         }
 
+        /// <summary>
+        /// Searches for the specified character within this slice.
+        /// </summary>
+        /// <returns>A value >= 0 if the character was found, otherwise &lt; 0</returns>
+        public int IndexOf(char c)
+        {
+            for (int i = Start; i <= End; i++)
+            {
+                if (Text[i] == c)
+                {
+                    return i;
+                }
+            }
+            return -1;
+        }
+
         /// <summary>
         /// Searches the specified text within this slice (matching lowercase).
         /// </summary>