Fixed codeplex issue 24176

This is a workaround for older versions of Mono that do not have System.Numerics.BigInteger.Parse.

The problem with the current implementation is that it is called directly in the method, which causes the runtime to resolve the types prior to executing the function, meaning that any call to ParseNumber will throw an exception, even if it is never used.

This patch simply moves the call to a separate method so the error only occurs if BigInteger functionality is actually required.