JSON.parse exception with multiple XBlocks in one unit

Using hotfix-2015-02-23

If a unit contains more than one XBlock instance that sets the json_args argument to initialize_js, an exception is thrown by JSON.parse when it tries to parse the concatenation of those JSON strings.

Steps to reproduce:
1. Install and enable any XBlock that passes a dict to xblock.fragment.Fragment#initialize_js, such as the attached
2. Create a Unit in Studio. Add two instances of that XBlock to this Unit.
3. Reload the page. Observe that JSON.parse complaints about an unexpected '{'
4. Load this Unit in LMS. Observe that JSON.parse complains about an unexpected character