Add and test for close event on Cursor

Despite (some) documentation to the contrary, some folks assume that destroying a stream will emit a close event. This Cursor does not emit a close event, which causes some interoperability issues. Streams should probably emit the close event despite not being required to do so by the streams2 spec.

See also pull-stream/stream-to-pull-stream#12.

lib/cursor.js

@@ -119,9 +119,11 @@ module.exports = class Cursor extends Readable {
           cursor = cursor[key](this._options[key]);
         });
 
+        cursor.on('close', (...args) => this.emit('close', ...args));
+
         this.cursor = cursor;
 
         return cursor;
       });
   }
-}
+}

test/cursor.js

@@ -229,7 +229,15 @@ describe('cursor', function() {
           resolve();
         });
     });
-  })
+  });
+
+  it('should emit a close event when closed', async () => {
+    const cursor = db.a.findAsCursor();
+    return new Promise((resolve) => {
+      cursor.once('close', resolve);
+      cursor.close();
+    });
+  });
 });
 
 
@@ -245,4 +253,4 @@ class ToArrayStream  extends Writable {
 
     cb();
   }
-}
+}