json.commonPath not working as expected

I'm running into some issues with the json api, I was having some trouble getting the 'child op' events to work correctly as the path that was passed in seemed to be wrong. That led me to json.commonPath:

Adding these tests to test/types/json.coffee

``` coffeescript
'commonPath - all but last': (test) ->
test.deepEqual 1, type.commonPath ['children', 0], ['children', 1]
test.done()

'commonPath - all match': (test) ->
test.deepEqual 3, type.commonPath ['children', 0, 'value'], ['children', 0, 'value']
test.done()

'commonPath - none match': (test) ->
test.deepEqual 0, type.commonPath ['key1', 0, 'value'], ['key2', 1, 'value']
test.done()

'commonPath - different': (test) ->
test.deepEqual 1, type.commonPath ['children', 0, 'value'], ['children', 1, 'value']
test.done()

'commonPath - different lengths': (test) ->
test.deepEqual 1, type.commonPath ['children'], ['children', 1, 'value']
test.deepEqual 1, type.commonPath ['children', 1, 'value'], ['children']
test.done()
```

Seems to show that commonPath isn't correctly returning the actual length of the common path, which is what it appears to be trying to do.

I've fixed this function but it breaks a lot of the other tests because transformComponent uses commonPath and seems to rely on it's current behaviour, so before I dive into transformComponent I want to check and see if I'm on the right track or if commonPath is behaving as expected.