Pop the 'parent' arg from ArrowItem opts parameter dict

If not, then an `KeyError: 'Invalid arrow style option "parent"'`
 is raised in setStyle.

pyqtgraph/graphicsItems/ArrowItem.py

@@ -17,7 +17,7 @@ class ArrowItem(QtGui.QGraphicsPathItem):
         the setStyle() method.
         """
         self.opts = {}
-        QtGui.QGraphicsPathItem.__init__(self, opts.get('parent', None))
+        QtGui.QGraphicsPathItem.__init__(self, opts.pop('parent', None))
 
         if 'size' in opts:
             opts['headLen'] = opts['size']

pyqtgraph/graphicsItems/tests/test_ArrowItem.py

@@ -0,0 +1,10 @@
+import pyqtgraph as pg
+
+app = pg.mkQApp()
+
+
+def test_ArrowItem():
+    parent = pg.GraphicsObject()
+    a = pg.ArrowItem(parent=parent, pos=(10, 10))
+    assert a.parentItem() is parent
+    assert a.pos() == pg.Point(10, 10)