Check blocked words against alphabet case-insensitively

Reflects sqids/sqids-spec#7

src/Sqids/SqidsEncoder.cs

@@ -110,8 +110,14 @@ public SqidsEncoder(SqidsOptions options)
 			StringComparer.OrdinalIgnoreCase // NOTE: Effectively removes items that differ only in casing — leaves one version of each word casing-wise which will then be compared against the generated IDs case-insensitively
 		);
 		options.BlockList.RemoveWhere(w =>
-			w.Length < 3 || // NOTE: Removes words that are less than 3 characters long
-			w.Any(c => !options.Alphabet.Contains(c)) // NOTE: Removes words that contain characters not found in the alphabet
+			// NOTE: Removes words that are less than 3 characters long
+			w.Length < 3 ||
+			// NOTE: Removes words that contain characters not found in the alphabet
+#if NETSTANDARD2_0
+			w.Any(c => options.Alphabet.IndexOf(c.ToString(), StringComparison.OrdinalIgnoreCase) == -1) // NOTE: A `string.Contains` overload with `StringComparison` didn't exist prior to .NET Standard 2.1, so we have to resort to `IndexOf` — see https://stackoverflow.com/a/52791476
+#else
+			w.Any(c => !options.Alphabet.Contains(c, StringComparison.OrdinalIgnoreCase))
+#endif
 		);
 		_blockList = options.BlockList.ToArray(); // NOTE: Arrays are faster to iterate than HashSets, so we construct an array here.