feat(AgmMarker): auto-convert string to Number for lat/lng (#1424)

fixes #771

Author: Dario Braun

packages/core/directives/marker.ts

@@ -158,6 +158,12 @@ export class AgmMarker implements OnDestroy, OnChanges, AfterContentInit {
 
   /** @internal */
   ngOnChanges(changes: {[key: string]: SimpleChange}) {
+    if (typeof this.latitude === 'string') {
+      this.latitude = Number(this.latitude);
+    }
+    if (typeof this.longitude === 'string') {
+      this.longitude = Number(this.longitude);
+    }
     if (typeof this.latitude !== 'number' || typeof this.longitude !== 'number') {
       return;
     }