checkRequired overrides email validation message.

Hi,
Following https://github.com/konsumer/mongoose-type-email/commit/2bd35bbdf4580551bc4d8fd2f90fae90c9e894e3 and its origin issue https://github.com/konsumer/mongoose-type-email/issues/24, there might be some change of behavior in the way error message are defined.

Considering this schema:
```js
const userSchema = mongoose.Schema({
email: {
type: mongoose.SchemaTypes.Email,
required: [true, "error.common.required"],
unique: true,
message: "error.common.email",
},
});
```

If an email is invalid for example `foobar` i'm expecting to get `error.common.email` as error message but i get `error.common.required`.
This is due to the fact that `checkRequired` does not only check for the mandatory value but also for the email validation: https://github.com/konsumer/mongoose-type-email/blob/07ca26a473124be5b717c3d44c7222f243ac409d/index.js#L35-L37
From my POV, `checkRequired` should focus only on requirement, leaving the email validation to the plugin later lifecycle.

Current code for checkRequired is :
```js
mongoose.SchemaTypes.Email.prototype.checkRequired = function (val) {
return typeof val === 'string' && validateEmail(val, this.options);
};
```

My current alternative is to override this function by setting back the default mongoose behavior:
```
mongoose.SchemaTypes.Email.prototype.checkRequired = function (val) {
return val != null;
};
```
That way, requirement is handled as before and will throw `error.common.required` only if value is missing. In case of incorrect email, `checkRequired` will be true, passing the requirement validation and will fail later in the plugin lifecycle providing `error.common.email` error as expected.

Beware that my alternative only care about non null value for requirement. It might be interesting to take care of `allowBlank` options here as well.

Start in index.js around the Email.prototype.checkRequired implementation and read the linked origin issue and commit for context. Verify how required and invalid-email values are reported, including the allowBlank option mentioned in the issue. Done means missing values retain the required message while invalid non-empty emails receive the email-validation message.

Written by the indexing model from the issue text.