Prepared statements should handle booleans properly

[kamipo]

Without this fix, booleans always be bound as 0.0 even if true.

Failures:

  1) Mysql2::Statement should handle booleans
     Failure/Error: expect(result.to_a).to eq(['true' => 1, 'false' => 0])

       expected: [{"true"=>1, "false"=>0}]
            got: [{"true"=>0.0 (#<BigDecimal:7fc532889ba0,'0.0',9(18)>), "false"=>0.0 (#<BigDecimal:7fc532889a88,'0.0',9(18)>)}]

       (compared using ==)

       Diff:
       @@ -1,2 +1,3 @@
       -[{"true"=>1, "false"=>0}]
       +[{"true"=>0.0 (#<BigDecimal:7fc532889ba0,'0.0',9(18)>),
       +  "false"=>0.0 (#<BigDecimal:7fc532889a88,'0.0',9(18)>)}]

     # ./spec/mysql2/statement_spec.rb:67:in `block (2 levels) in <top (required)>'