HTTP headers are not case-sensitive

[Douglas Bergami]

supertest.get('test').set('Cache-Control', 'xablau' )
comes as

headers: {
'cache-control: xablau'
}

headers are always converted to lowercase

how can i keep it exactly as it was set

[Jono]

This seems to be an old issue, #350 #796, not sure why it's still happening (it's happening for me as well)