Fixed bug in inline Specification

[SathishN]

When creating Specification with inline specification. it throws ArgumentNullException.

public class Person
{
    public string Name { get; set; }

    public Address2 MailingAddress { get; set; }

    public Address2 ShippingAddress { get; set; }
}

public class Address2
{
    public const string MailingType = "MAIL";

    public const string ShippingType = "SHIP";

    public string Type { get; set; }
    public string AddressLine { get; set; }
    public string City { get; set; }
    public string State { get; set; }
}

public class PersonSpecification : Validates<Person>
{
    public PersonSpecification()
    {
        Check(_ => _.Name).Required();

        Check(_ => _.MailingAddress).Optional()
            .Specification(v => v.Check(x => x.Type == Address2.MailingType))
            .And
            .Specification<Address2Specification>();

        Check(_ => _.ShippingAddress).Optional()
            .Specification(v => v.Check(x => x.Type == Address2.ShippingType))
            .And
            .Specification<Address2Specification>();
    }
}

public class Address2Specification : Validates<Address2>
{
    public Address2Specification()
    {
        Check(_ => _.AddressLine).Required();
        Check(_ => _.City).Required();
        Check(_ => _.State).Required();
    }
}

that throws following error

System.ArgumentNullException : Value cannot be null.
Parameter name: key
   at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
   at System.Collections.Generic.Dictionary`2.ContainsKey(TKey key)
   at SpecExpress.Rules.RuleValidatorContext.GetValidationProperty(PropertyValidator validator, Type instanceType) in RuleValidatorContext.cs: line 96
   at SpecExpress.Rules.RuleValidatorContext.SelectPropertyName(Object instance, PropertyValidator validator) in RuleValidatorContext.cs: line 55
   at SpecExpress.Rules.RuleValidatorContext..ctor(Object instance, PropertyValidator validator, RuleValidatorContext parentContext) in RuleValidatorContext.cs: line 36
   at SpecExpress.Rules.RuleValidatorContext`2..ctor(T instance, PropertyValidator`2 validator, RuleValidatorContext parentContext) in RuleValidatorContext.cs: line 114
   at SpecExpress.PropertyValidator`2.Validate(T instance, RuleValidatorContext parentRuleContext, SpecificationContainer specificationContainer, ValidationNotification notification) in PropertyValidator.cs: line 469
   at SpecExpress.PropertyValidator`1.Validate(Object instance, RuleValidatorContext parentRuleContext, SpecificationContainer specificationContainer, ValidationNotification notification) in PropertyValidator.cs: line 207
   at SpecExpress.Rules.GeneralValidators.SpecificationRule`2.Validate(RuleValidatorContext`2 context, SpecificationContainer specificationContainer, ValidationNotification notification) in SpecificationRule.cs: line 59
   at lambda_method(Closure, RuleValidatorContext`2, SpecificationContainer, ValidationNotification)
   at SpecExpress.PropertyValidator`2.Validate(T instance, RuleValidatorContext parentRuleContext, SpecificationContainer specificationContainer, ValidationNotification notification) in PropertyValidator.cs: line 477
   at SpecExpress.PropertyValidator`1.Validate(Object instance, SpecificationContainer specificationContainer, ValidationNotification notification) in PropertyValidator.cs: line 202
   at SpecExpress.SpecificationBase.Validate(Object instance, SpecificationContainer specificationContainer, ValidationNotification notification) in SpecificationBase.cs: line 34
   at SpecExpress.ValidationCatalog.Validate(Object instance, SpecificationContainer container, SpecificationBase specificationBase) in ValidationCatalog.cs: line 259
   at SpecExpress.ValidationCatalog.Validate(Object instance) in ValidationCatalog.cs: line 275
   at SpecExpress.Test.PersonSpecificationTests.Valid_Person() in PersonSpecificationTests.cs: line 44

Fixes are in the pull request.