Rename variable that holds whether or not the class should validate a float number
Refactor to avoid earlier returns
Stop using local variables everywhere, make use of the reader
Avoid a hash creation since defaults is a new hash already
No need for #tap
Remove useless empty string
Make both conversion methods work similarly

The conversion without area code already changed the passed number in
place, so change the other method to do the same.
Change delimiter check order: first check if it is present

This reads a lot better, and we won't need to try start_with? for blank
delimiters.

Also rename method name to read better.
Remove earlier return in favor of conditional