Fix issue with long.MaxValue + added tests

[Falco20019]

Fixes #673 by using unchecked instead of checked cast. Also added test cases for avoiding regression.

[EdwardCooke]

The issue I see with this is if someone gets a number greater than long.MaxValue (9223372036854775807) it will no longer be correct.

Example:

Console.WriteLine(unchecked(-(long)9223372036854775809));

results in 9223372036854775807

As such, I dont think we want to do this.

[EdwardCooke]

One way to fix it would be to check if the number is the positive of the negative and if so, return long.minvalue.

            if (isNegative)
            {
                long toCast;

                // the only real difference between long.minvalue and maxvalue is 1.
                // since the number part of long.minvalue is larger than long.maxvalue
                // we will check to see if it is the number and then return the min value
                // otherwise we will run it through checked to validate it is within our range
                if (result == 9223372036854775808)
                {
                    toCast = long.MinValue;
                }
                else
                {
                    toCast = checked(-(long)result);
                }

                return CastInteger(toCast, typeCode);
            }
            else
            {
                return CastInteger(result, typeCode);
            }
        }

[EdwardCooke]

I've used parts of your tests but replaced the logic with what I posted above. Closing this in preference of #702.