Well let see what the w3c spec says about the  `<nav>`:
*The nav element represents a section of a document that links to other documents or to parts within the document itself; that is, a section of navigation links.*
And about the `<header>`:
*The header element represents the header of a section.*

The `<nav>` definition doesn't fits with the usage we do with our current `<header>`.
So i believe that keeping `<header>` tag is reasonable