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

src/db/disposable.cr

@@ -8,8 +8,8 @@ module DB
     # Closes this object.
     def close
       return if @closed
-      @closed = true
       do_close
+      @closed = true
     end
 
     # Returns `true` if this object is closed. See `#close`.
@@ -18,6 +18,7 @@ module DB
     end
 
     # Implementors overrides this method to perform resource cleanup
+    # If an exception is raised, the resource will not be marked as closed.
     protected abstract def do_close
   end
 end