BigDecimal should accept a Float value without a precision

[mrkn]

irb(main):002:0> BigDecimal(5.1)
(irb):2:in `BigDecimal': can't omit precision for a Float. (ArgumentError)
        from (irb):2:in `<main>'
        from /home/mrkn/.rbenv/versions/3.0.2/lib/ruby/gems/3.0.0/gems/irb-1.3.8.pre.9/exe/irb:11:in `<top (required)>'
        from /home/mrkn/.rbenv/versions/3.0/bin/irb:23:in `load'
        from /home/mrkn/.rbenv/versions/3.0/bin/irb:23:in `<main>'

This should works like below.

irb(main):003:0> BigDecimal(5.1, Float::DIG+1)
=> 0.51e1

~This is necessary to resolve #212~.