BigDecimal encoded differently in compat mode

In compat mode, BigDecimals are encoded as strings. In other modes, they are numbers. This is probably a mistake, because the correct code appears to be accidenatlly commented out in 7a573f2810dbd33ed163bde91b00bff333a328bb.

This PR reverts it and fixes the problem.

    :::ruby
    Oj.dump(BigDecimal.new("7"), mode: :strict)
    => 0.7E1
    Oj.dump(BigDecimal.new("7"), mode: :object)
    => 0.7E1
    Oj.dump(BigDecimal.new("7"), mode: :compat)
    => "0.7E1"