Slight modification to WebHost class

[stanislavstoyanov99]

I have made a slight improvement of reflection code in WebHost class, which enables us for future exams and apps that will use the framework to use nullable primitive types such as double, decimal. The problem I encountered was hitted while doing the exam preparation of the course - we cannot use nullable double/decimal for price field. If we try to omit the field with price, for example, the MVC Framework crashes. The modification I have made enables to check in Controller for nullable types except string and process the error how we want.