ensure the connection is released after exec

avoid releasing connection twice on exec.
avoid releasing connection when QueryMethod#query is blockless called.

spec/dummy_driver.cr

@@ -49,6 +49,7 @@ class DummyDriver < DB::Driver
 
     protected def perform_exec(args : Enumerable)
       set_params args
+      raise "forced exception due to query" if @query == "raise"
       DB::ExecResult.new 0i64, 0_i64
     end
 

spec/dummy_driver_spec.cr

@@ -237,6 +237,14 @@ describe DummyDriver do
       end
     end
 
+    it "should raise executing raise query" do
+      with_dummy do |db|
+        expect_raises do
+          db.exec "raise"
+        end
+      end
+    end
+
     {% for value in [1, 1_i64, "hello", 1.5, 1.5_f32] %}
       it "should set positional arguments for {{value.id}}" do
         with_dummy do |db|

spec/statement_spec.cr

@@ -118,4 +118,13 @@ describe DB::Statement do
       rs.statement.should be(stmt)
     end
   end
+
+  it "connection should be released if error occurs during exec" do
+    with_dummy do |db|
+      expect_raises do
+        db.exec "raise"
+      end
+      db.@in_pool.should be_true
+    end
+  end
 end