Avoid generating email/domain_word with non-ascii

In ja and other locales using non-ascii characters,
generating emails and domains from the faker name
results in non-ascii emails and domains.

Such email and domains are invalid, because they contain non-ascii
characters.

```
Faker::Config.locale = :ja
Faker::Internet.email #=> "@.biz"
```

So, if first_name and last_name is non-ascii characters,
we use fist_name and last_name in locale :en.