Consecutive `_` is not allowed as a numeric expression:

  1_2__3             #=> SyntaxError
  Float("1_2__3")    #=> ArgumentError
  Integer("1_2__3")  #=> ArgumentError
  "1_2__3".to_i      #=> 12

But `String#to_f` accept it, so I fixed the issue.

Before this patch:

  "1_2__3".to_f      #=> 123

After this patch:

  "1_2__3".to_f      #=> 12