Added method in ParseUtil that strips out every invalid XML character from a string.

Added test for ParseUtil.RemoveInvalidXmlChars() using a snippet of rss from XSeeds that was originally causing problems.

src/Jackett.Test/Util/ParseUtilTests.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Xml.Linq;
+using System.Xml.XPath;
+using FluentAssertions;
+using Jackett.Utils;
+using NUnit.Framework;
+
+namespace JackettTest.Util
+{
+    [TestFixture]
+    public class ParseUtilTests
+    {
+        private static string InvalidRssXml
+        {
+            get
+            {
+                var type = typeof(ParseUtilTests);
+                using (var resourceStream = type.Assembly.GetManifestResourceStream($"{type.Namespace}.Invalid-RSS.xml"))
+                using (var sr = new StreamReader(resourceStream))
+                {
+                    return sr.ReadToEnd();
+                }
+                
+            }
+        }
+
+        [Test]
+        public void Invalid_RSS_should_parse_after_removing_invalid_chars()
+        {
+            var invalidRss = InvalidRssXml;
+            Action parseAction = () => XDocument.Parse(invalidRss);
+            parseAction.ShouldThrow<Exception>().WithMessage("'\a', hexadecimal value 0x07, is an invalid character. Line 12, position 7.");
+
+            var validRSs = ParseUtil.RemoveInvalidXmlChars(invalidRss);
+            var rssDoc = XDocument.Parse(validRSs);
+            rssDoc.Root.Should().NotBeNull();
+            var description = rssDoc.Root.XPathSelectElement("//description");
+            description.Value.Should().Contain("Know Your Role and Shut Your Mouth!");
+        }
+    }
+}