feat($interpolate): use custom toString member if present

BREAKING CHANGE: use custom toString implementation when present on object value types.
Behavior is consistent with other implementations.

Closes angular#7317

Author: danielkrainas

src/ng/interpolate.js

@@ -272,7 +272,11 @@ function $InterpolateProvider() {
               break;
             }
             default: {
-              value = toJson(value);
+              if(typeof value.toString !== 'undefined' && value != null && value.toString !== Object.prototype.toString && !isArray(value)) {
+                value = value.toString();
+              } else {
+                value = toJson(value);
+              }
             }
           }