U4-2635 Skip past replaced word when checking

Avoids infinite loop if search and replacement are equal

src/Umbraco.Core/StringExtensions.cs

@@ -1191,11 +1191,13 @@ namespace Umbraco.Core
         /// <returns>Updated string</returns>
         public static string Replace(this string source, string oldString, string newString, StringComparison stringComparison)
         {
-            // Initialised to zero so it can be used as startIndex on first iteration.
-            int index = 0;
+            // This initialisation ensures the first check starts at index zero of the source. On successive checks for
+            // a match, the source is skipped to immediately after the last replaced occurrence for efficiency
+            // and to avoid infinite loops when oldString and newString compare equal. 
+            int index = -1 * newString.Length;
 
-            // Determine if there are any matches left in source, starting from last found match.
-            while((index = source.IndexOf(oldString, index, stringComparison)) >= 0)
+            // Determine if there are any matches left in source, starting from just after the result of replacing the last match.
+            while((index = source.IndexOf(oldString, index + newString.Length, stringComparison)) >= 0)
             {
                 // Remove the old text.
                 source = source.Remove(index, oldString.Length);