Convinence header methods like authorization should accept DslProperty

[jakub-bochenski]

Describe the bug
This doesn't work

                    authorization $(
                            p(nonEmpty()),
                            c(nonEmpty())
                    )

'authorization' in 'org. springframework. cloud. contract. spec. internal. Headers' cannot be applied to '(org. springframework. cloud. contract. spec. internal. DslProperty)'
Workaround

                    header 'Authorization', $(
                            p(nonEmpty()),
                            c(nonEmpty())
                    )