#104 @AssertType rejects a promise when used in async methods

[HitoriSensei]

I've prepared a PR addressing #104

Changes are reflected in tests and in readme:

async and Promise returning methods

AssertType can also work correctly with async methods, returning promise rejected with TypeGuardError

To enable this functionality, you need to emit decorators metadata for your TypeScript project.

{
    "compilerOptions": {
      "emitDecoratorMetadata": true
    }
}

Then AssertType will work with async methods and Promise returning methods automatically.

import { ValidateClass, AssertType } from 'typescript-is';

@ValidateClass()
class A {
    async method(@AssertType({ async: true }) value: number) {
        // You can safely use value as a number
        return value;
    }

    methodPromise(@AssertType({ async: true }) value: number): Promise<number> {
        // You can safely use value as a number
        return Promise.resolve(value);
    }
}

new A().method(42).then(value => value === 42 /* true */); 
new A().method('42' as any).catch(error => {
    // error will be of TypeGuardError type
})
new A().methodPromise('42' as any).catch(error => {
    // error will be of TypeGuardError type
})

If you want to throw synchronously for some reason, you can override the behaviour using with @AssertType({ async: false }):

import { ValidateClass, AssertType } from 'typescript-is';

@ValidateClass()
class A {
    async method(@AssertType({ async: false }) value: number) {
        // You can safely use value as a number
        return value;
    }
}

new A().method(42).then(value => value === 42 /* true */);
new A().method('42' as any); // will throw error

If you cannot or don't want to enable decorators metadata, you still make AssertType reject with promise using @AssertType({ async: true })

import { ValidateClass, AssertType } from 'typescript-is';

@ValidateClass()
class A {
    async method(@AssertType({ async: true }) value: number) {
        // You can safely use value as a number
        return value;
    }
}