Fix audited request header lookup (#3707)
Fix audited request header lookup (#3707)

The headers are stored lowercased but the lookup function wasn't
properly lowercasing when indexing in the header map.

Fixes #3701
hashicorp/vault,hashicorp/vault,rnaveiras/vault,Aloomaio/vault,hashicorp/vault,joelthompson/vault,kingland/vault,Aloomaio/vault,Aloomaio/vault,joelthompson/vault,kingland/vault,naunga/vault,kingland/vault,hartsock/vault,hashicorp/vault,rnaveiras/vault,hartsock/vault,hashicorp/vault,mgk/vault,naunga/vault,hartsock/vault,hashicorp/vault,rnaveiras/vault,quixoten/vault,hartsock/vault,hartsock/vault,rnaveiras/vault,kingland/vault,quixoten/vault,mgk/vault,Aloomaio/vault,quixoten/vault,quixoten/vault,mgk/vault,joelthompson/vault,rnaveiras/vault,joelthompson/vault,mgk/vault,kingland/vault