client: remove custom "headers" type, and use "http.Header" instead

Use http.Header, which is more descriptive on intent, and we're already
importing the package in the client. Removing the "header" type also fixes
various locations where the type was shadowed by local variables named
"headers".

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/container_attach.go

@@ -2,6 +2,7 @@ package client // import "github.com/docker/docker/client"
 
 import (
 	"context"
+	"net/http"
 	"net/url"
 
 	"github.com/docker/docker/api/types"
@@ -52,8 +53,7 @@ func (cli *Client) ContainerAttach(ctx context.Context, container string, option
 		query.Set("logs", "1")
 	}
 
-	headers := map[string][]string{
+	return cli.postHijacked(ctx, "/containers/"+container+"/attach", query, nil, http.Header{
 		"Content-Type": {"text/plain"},
-	}
-	return cli.postHijacked(ctx, "/containers/"+container+"/attach", query, nil, headers)
+	})
 }