Fix JSON.unsafe_load with proc argument

[robinetmiller]

Commit 73d2137 (released with version 2.11.0) removes JSON.recurse_proc and its the call in JSON.load, opting instead to replace the call with the :on_load option. However, this is not the only instance of recurse_proc, and anyone using JSON.unsafe_load with a proc will receive a NoMethodError.

It would appear that JSON.unsafe_load had no tests whatsoever, causing this failure.

These commits:

1. Add tests covering JSON.unsafe_load
2. AlignJSON.unsafe_load with the current behaviour of JSON.load
3. Update the documentation to show that the proc must return the object, as the examples as is do not work.