[FIX] web: pass bin_size in context

When fetching record, `bin_size` needs to be passed in the context
to notify the server to send the `bin_size` and not the whole base64
of binary fields.

addons/web/static/src/js/views/basic/basic_model.js

@@ -1253,8 +1253,7 @@ var BasicModel = AbstractModel.extend({
                 model: record.model,
                 method: 'read',
                 args: [[record.res_id], fieldNames],
-                context: record.context,
-                // context: {bin_size: true} // FIXME: when editing a subrecord in the partner form view, it tries to write the bin_size on the image field
+                context: _.extend({}, record.context, {bin_size: true}),
             })
             .then(function (result) {
                 result = result[0];

addons/web/static/tests/views/form_tests.js

@@ -161,8 +161,8 @@ QUnit.module('Views', {
         form.destroy();
     });
 
-    QUnit.test('only necessary fields are fetched', function (assert) {
-        assert.expect(1);
+    QUnit.test('only necessary fields are fetched with correct context', function (assert) {
+        assert.expect(2);
 
         var form = createView({
             View: FormView,
@@ -177,6 +177,8 @@ QUnit.module('Views', {
                 // field, not sure why.  Maybe this test should be modified.
                 assert.deepEqual(args.args[1], ["foo", "display_name"],
                     "should only fetch requested fields");
+                assert.deepEqual(args.kwargs.context, {bin_size: true},
+                    "bin_size should always be in the context");
                 return this._super(route, args);
             }
         });
@@ -3142,8 +3144,8 @@ QUnit.module('Views', {
             },
             mockRPC: function (route, args) {
                 if (args.method === 'read') {
-                    assert.deepEqual(args.kwargs.context, {some_context: false},
-                        "record's context shouldn't have been modified");
+                    assert.strictEqual('some_context' in args.kwargs.context && !args.kwargs.context.some_context, true,
+                        "the context should have been set");
                 }
                 return this._super.apply(this, arguments);
             },