AVG function returns always integer class value

[a5-stable]

I am having trouble getting an accurate result when using the AVG function with decimal values。
For example, when I run AVG(2, 3),

c = Dentaku::Calculator.new
c.evaluate!("AVG(2, 3)")

# expected value
#=> 2.5

# actual value
#=> 2

I think that the result should be 2.5.

[a5-stable]

I made PR #294 to fix this.
If you have some free time, please take a look and let me know your thoughts, thank you 🙇‍♀️

[a5-stable]

the fix was merged, thank you 🙇‍♀️