Fix http-json plugin by always converting expected value to string

[NicolasLM]

Hi Thibault,

Thank you for your contribution. It looks legit, however could you add a simple test case for this? You can add it there https://github.com/NicolasLM/sauna/blob/master/tests/test_plugins.py#L638

[thbkrkr]

Of course! Here is the test.

[NicolasLM]

Thanks, do you need a release on PyPI?

[thbkrkr]

It's not necessary. I run sauna in Docker by cloning a specific tag/sha1. Thanks.