fixed fiber middleware header issue

[dani3lsf]

Fix for issue: https://github.com/deepmap/oapi-codegen/issues/1621

[oliveromahony]

@dani3lsf @alexandear is there any updates on this? It is impacting fiber generation for my headers

[alexandear]

@dani3lsf @alexandear is there any updates on this? It is impacting fiber generation for my headers

This PR requires a review from maintainers.

[seth2810]

Hey @dani3lsf @alexandear, The fix could be even simpler, because as it stated in fiber@v2 docs ctx.Get returns single header and it's string not slice type:

Get - Returns the HTTP request header specified by the field.