Complex.ToString must not pass format string to string.Format

Type: System.Numerics.Complex

IFormattable format strings and string.Format formats are different
and incompatible. For example, `x.ToString("G")` and `x.ToString(null)`
must be equivalent to `x.ToString()`, but they behave very differently
with string.Format, where the former just returns "G" and the latter
throws an ArgumentNullException.

Changes System.Numerics.Complex.ToString to pass the format string to
double.ToString instead of string.Format.

Adds UnitTests for the changes in new ComplexTest.cs file