Unable to update path containing @ since sjson 1.2.0

[dunglas]

Hi,

First thanks for this great library.

Since version 1.2, if the path contains a @, it fails to update the document. This is similar to #61 but it's not resolved in version 1.2.5.

To reproduce, you can clone Vulcain and run this test: https://github.com/dunglas/vulcain/blob/f2d77e93a75ea803607bbc816c7eb1d534a2e7bc/server_test.go#L191

With version 1.1.7, it's green, but starting with version 1.2.0 it's red.

I hope this helps.

[dunglas]

As a workaround, escaping special sjson chars fixes the issue: dunglas/vulcain#104