Fixed bug in removing trailing delimiter

The existing code strips all trailing delimiters which is incorrect, it should only remove the final delimiter.  The problem occurs when the last column is empty.  Say you export three fields, first name, last name & email (where email is null).  Expected output would be "John;Doe;" but instead you get "John;Doe" which is problematic.

Author: mguinness

src/WebApiContrib.Core.Formatter.Csv/CsvOutputFormatter.cs

@@ -149,7 +149,7 @@ public async override Task WriteResponseBodyAsync(OutputFormatterWriteContext co
                     }
                 }
 
-                await streamWriter.WriteLineAsync(valueLine.TrimEnd(_options.CsvDelimiter.ToCharArray()));
+                await streamWriter.WriteLineAsync(valueLine.Remove(valueLine.Length - _options.CsvDelimiter.Length));
             }
 
             await streamWriter.FlushAsync();