Use full header type in response header instances

This is required to be able provide `Description` into headers, for
example like this:
```
type PaginationTotalCountHeader =
  Header'
    '[ Description "Indicates to the client total count of items in collection"
     , Optional
     , Strict
     ]
    "Total-Count"
    Int
```

servant/src/Servant/API/ResponseHeaders.hs

@@ -37,7 +37,7 @@ module Servant.API.ResponseHeaders
 import           Control.DeepSeq
                  (NFData (..))
 import           Data.ByteString.Char8     as BS
-                 (ByteString, init, pack, unlines)
+                 (ByteString, pack)
 import qualified Data.CaseInsensitive      as CI
 import qualified Data.List                 as L
 import           Data.Proxy
@@ -52,7 +52,7 @@ import           Web.HttpApiData
 import           Prelude ()
 import           Prelude.Compat
 import           Servant.API.Header
-                 (Header)
+                 (Header, Header')
 import           Servant.API.UVerb.Union
 import qualified Data.SOP.BasicFunctors as SOP
 import qualified Data.SOP.NS as SOP
@@ -130,7 +130,7 @@ instance GetHeadersFromHList '[] where
     getHeadersFromHList _ = []
 
 instance (KnownSymbol h, ToHttpApiData x, GetHeadersFromHList xs)
-    => GetHeadersFromHList (Header h x ': xs)
+    => GetHeadersFromHList (Header' mods h x ': xs)
   where
     getHeadersFromHList hdrs = case hdrs of
         Header val `HCons` rest          -> (headerName , toHeader val) : getHeadersFromHList rest
@@ -151,7 +151,7 @@ instance GetHeaders' '[] where
     getHeaders' _ = []
 
 instance (KnownSymbol h, GetHeadersFromHList rest, ToHttpApiData v)
-    => GetHeaders' (Header h v ': rest)
+    => GetHeaders' (Header' mods h v ': rest)
   where
     getHeaders' hs = getHeadersFromHList $ getHeadersHList hs