fix #1. avoid marking as closed if do_close raise exception

spec/disposable_spec.cr

@@ -0,0 +1,31 @@
+require "./spec_helper"
+
+class ADisposable
+  include DB::Disposable
+  @raise = false
+
+  property raise
+
+  protected def do_close
+    raise "Unable to close" if @raise
+  end
+end
+
+describe DB::Disposable do
+  it "should mark as closed if able to close" do
+    obj = ADisposable.new
+    obj.closed?.should be_false
+    obj.close
+    obj.closed?.should be_true
+  end
+
+  it "should not mark as closed if unable to close" do
+    obj = ADisposable.new
+    obj.raise = true
+    obj.closed?.should be_false
+    expect_raises Exception do
+      obj.close
+    end
+    obj.closed?.should be_false
+  end
+end