fails:String#to_f treats leading characters of self as a floating point number
fails:String#to_f treats special float value strings as characters
fails:String#to_f allows for underscores, even in the decimal side
fails:String#to_f returns 0 for strings with any non-digit in them
fails:String#to_f returns 0 for strings with leading underscores
fails:String#to_f takes an optional sign
fails:String#to_f returns 0.0 if the conversion fails
