Do `json_encode` for quote string and not just replace “ to \”

Same unstable escaping pattern (replace `"` with `\"` and wrap with `"`) was also used in `SahiClient`. To minimize amount of performed changes I've decided just to call `json_encode` from within `prepareXPath` and remove `"`, that are added around the result automatically.

Fixes #30.

__Changes from #33 needs to be merged first. Until it's done, the `testAdvancedForm` will fail here.__