utils.mergeObjects: an argument is missing in a function call

When calling the gecko version of mergeObjects, options were missing.
So a test about keepReferences failed with SlimerJS & gecko25+...
utils.mergeObjects+gecko: fix copy of properties with getter only

if a property is only a getter, we could have a Javascript error in strict mode when
setting the value to the new object.

"TypeError: setting a property that has only a getter"

We should then define the property with defineProperty, not by setting the value on the
new object.
Merge pull request #746 from laurentj/fix-mergeobjects

Fix mergeobjects