add check for whitespace or newline characters to existing token validation

Checks to see if a token contains any illegal characters, like whitespace or a newline. If it is, throws an ArgumentException warning the user that their token may be invalid.

I considered only checking the first and last character, but given that a token containing whitespace or a newline wouldn't work either I figured this made sense.

src/Discord.Net.Core/Utils/TokenUtils.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Text;
 
 namespace Discord
@@ -119,6 +121,25 @@ namespace Discord
             return DecodeBase64UserId(segments[0]).HasValue;
         }
 
+        /// <summary>
+        ///     The set of all characters that are not allowed inside of a token.
+        /// </summary>
+        internal static char[] IllegalTokenCharacters = new char[]
+        {
+            ' ', '\t', '\r', '\n'
+        };
+
+        /// <summary>
+        ///     Checks if the given token contains a whitespace or newline character
+        ///     that would fail to log in.
+        /// </summary>
+        /// <param name="token"> The token to validate. </param>
+        /// <returns>
+        ///     True if the token contains a whitespace or newline character.
+        /// </returns>
+        internal static bool CheckContainsIllegalCharacters(string token)
+            => token.IndexOfAny(IllegalTokenCharacters) != -1;
+
         /// <summary>
         ///     Checks the validity of the supplied token of a specific type.
         /// </summary>
@@ -131,6 +152,9 @@ namespace Discord
             // A Null or WhiteSpace token of any type is invalid.
             if (string.IsNullOrWhiteSpace(token))
                 throw new ArgumentNullException(paramName: nameof(token), message: "A token cannot be null, empty, or contain only whitespace.");
+            // ensure that there are no whitespace or newline characters
+            if (CheckContainsIllegalCharacters(token))
+                throw new ArgumentException(message: "The token contains a whitespace or newline character. Ensure that the token has been properly trimmed.", paramName: nameof(token));
 
             switch (tokenType)
             {

test/Discord.Net.Tests/Tests.TokenUtils.cs

@@ -99,6 +99,16 @@ namespace Discord
         [InlineData("937it3ow87i4ery69876wqire")]
         // 57 char bot token
         [InlineData("MTk4NjIyNDgzNDcxOTI1MjQ4.Cl2FMQ.ZnCjm1XVW7vRze4b7Cq4se7kK")]
+        // ends with invalid characters
+        [InlineData("MTk4NjIyNDgzNDcxOTI1MjQ4.Cl2FMQ.ZnCjm1XVW7vRze4b7Cq4se7k ")]
+        [InlineData("MTk4NjIyNDgzNDcxOTI1MjQ4.Cl2FMQ.ZnCjm1XVW7vRze4b7Cq4se7k\n")]
+        [InlineData("MTk4NjIyNDgzNDcxOTI1MjQ4.Cl2FMQ.ZnCjm1XVW7vRze4b7Cq4se7k\t")]
+        [InlineData("MTk4NjIyNDgzNDcxOTI1MjQ4.Cl2FMQ.ZnCjm1XVW7vRze4b7Cq4se7k\r\n")]
+        // starts with invalid characters
+        [InlineData(" MTk4NjIyNDgzNDcxOTI1MjQ4.Cl2FMQ.ZnCjm1XVW7vRze4b7Cq4se7k")]
+        [InlineData("\nMTk4NjIyNDgzNDcxOTI1MjQ4.Cl2FMQ.ZnCjm1XVW7vRze4b7Cq4se7k")]
+        [InlineData("\tMTk4NjIyNDgzNDcxOTI1MjQ4.Cl2FMQ.ZnCjm1XVW7vRze4b7Cq4se7k")]
+        [InlineData("\r\nMTk4NjIyNDgzNDcxOTI1MjQ4.Cl2FMQ.ZnCjm1XVW7vRze4b7Cq4se7k")]
         [InlineData("This is an invalid token, but it passes the check for string length.")]
         // valid token, but passed in twice
         [InlineData("MTk4NjIyNDgzNDcxOTI1MjQ4.Cl2FMQ.ZnCjm1XVW7vRze4b7Cq4se7kKWsMTk4NjIyNDgzNDcxOTI1MjQ4.Cl2FMQ.ZnCjm1XVW7vRze4b7Cq4se7kKWs")]