Some tests

[chesedo]

I recently did custom modifications of this plugin for a client. Thought you might benefit from some tests I've created in the process.

The following tests are added:

• build_url - which refactors this code into a function to test it
• build_response

I also have a test for vendor_params::from_header_array but that changes the Dockerfile and build.rs quite a bit to remove the MRCP_INCLUDE_PATH env variable which cargo test does not detect. Will you be ok with changing these two files?

Technically a test step can also be added to github actions...

[adeschamps]

Thank you for the contribution! At a glance this looks great. I'll take a closer look in the next day or two, and then merge this.

[adeschamps]

This is great, thank you.

As for the other test you mentioned -- I'm totally open to changes to Dockerfile and build.rs. The include path env variable works on the systems that I've tested with (Ubuntu and CentOS) but if a more flexible and/or portable setup is possible, then I'd gladly accept a contribution.