Support single header built-in testing function

[theredfish]

Continue to improve the API so we can test a single header and provide an expression as second argument:

.header("content-type", is("application/json; charset=utf-8"))

this HOF format was first found with #25 - I have an ongoing implementation where I'm able to provide an expression in an expression. Here we are at a level before, where header doesn't take one Expression argument but a header key followed by an Expression.

We are following the principle of composition, this one evolved since the first versions and the code could become a bit messy due to consecutive API updates. Some refacto could be necessary.