Rename Internet#user_name to #username

[tylerhunt]

Call me pedantic, but this—by far—has always been my biggest gripe with Faker. "Username" is the more typical and standardized spelling. I always seem to end up with code like this:

User.new(
  username: Faker::Internet.user_name,
  password: Faker::Internet.password,
)

Having to remember that difference for this one library (and put up with the misalignment to boot 😉) is a constant source of frustration. Here's what that code would look like now:

User.new(
  username: Faker::Internet.username,
  password: Faker::Internet.password,
)

Ah, much improved!

I've added an alias to maintain backwards compatibility.

[stympy]

Thanks 😉