evas proxy: make it work for File_Save.save

evas proxy: make it work for File_Save.save

Summary:
File_Save.save does not work for proxy object from following commit.

   c53f152 evas: Make save() work on snapshots

Test Plan:
1. Add an image object and set source object.
evas_object_image_source_set(obj, source);

2. Save the object as a file when you need.
evas_object_image_save(obj, "./file_name.png", NULL, NULL);

Reviewers: cedric, Hermet, jsuya

Reviewed By: Hermet

Subscribers: zmike, subodh6129, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D10629