add support for undefined value in isOptional

[codebycorey]

fixes #101

[codebycorey]

@pleerock I was wondering if you would look over this PR

[adnan-kamili]

Shouldn't undefined be treated differently as it is not same as empty string or null. Suppose if property is of type boolean it would allow empty string or null to bypass the validation, though they are incorrect (say it is being stored in database), I think there should be a new decorator @IsNotRequired() which explicitly ignores validation if value is only undefined. Or if you want to follow semantics of Typescript, functionality of @IsOptional() and @IsNotRequired() can be switched with above semantics, which would be a breaking change.

[kavinvin]

As @adnan-kamili said, why does @IsOptional also treat empty string as a missing value while it's actually defined and is certainly doesn't missing but only left empty? Moreover, it allows many validation hole; for example, if I use @Contains which checks for substring and then add @Optional, when a user enter an empty string it will be qualified as a valid input.

And it would be great if it both @IsOptional and @IsDefined are each other counterpart by making @IsDefined bypass all other validation like@IsOptional does; as in practice, both null and undefined won't pass any other validation anyway. It's strange if it also output error "number must be larger than 200" while the user doesn't even fill a certain property (which means undefined) or sending null value.
https://github.com/pleerock/class-validator/blob/master/src/validation/Validator.ts#L261

Added: Also, I wonder if @IsDefined should also checks null above from undefined because null is still has a meaning if you mean to keep it in database, while null can mean unknown or hasn't yet been entered by user, undefined means value is missing or unspecified (because of some error or technical intention; like, if you want to left the field use its default value or don't want to update the field). And hence its name @IsDefined, should nullchecking be considered to separate itself to @IsNotNull?

[adnan-kamili]

@pleerock allowing null or empty string to bypass in @IsOptional is a security hole, it would allow null or "" values to bypass for every datatype like boolean, Date, number. To use this decorator one needs to add additional validation checks in the code which sort of defeats the purpose.

[codebycorey]

@pleerock You merge but never released it to npm.
Anyway you can do that please.

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