fix(AgmCircle): dragstart event misspelled

Minor change to casing on drag start event

Fixes #1537

Author: IsaacSomething

packages/core/directives/circle.ts

@@ -206,7 +206,7 @@ export class AgmCircle implements OnInit, OnChanges, OnDestroy {
     events.set('dblclick', this.circleDblClick);
     events.set('drag', this.drag);
     events.set('dragend', this.dragEnd);
-    events.set('dragStart', this.dragStart);
+    events.set('dragstart', this.dragStart);
     events.set('mousedown', this.mouseDown);
     events.set('mousemove', this.mouseMove);
     events.set('mouseout', this.mouseOut);