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/image_push.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"io"
+	"net/http"
 	"net/url"
 
 	"github.com/docker/distribution/reference"
@@ -50,6 +51,7 @@ func (cli *Client) ImagePush(ctx context.Context, image string, options types.Im
 }
 
 func (cli *Client) tryImagePush(ctx context.Context, imageID string, query url.Values, registryAuth string) (serverResponse, error) {
-	headers := map[string][]string{registry.AuthHeader: {registryAuth}}
-	return cli.post(ctx, "/images/"+imageID+"/push", query, nil, headers)
+	return cli.post(ctx, "/images/"+imageID+"/push", query, nil, http.Header{
+		registry.AuthHeader: {registryAuth},
+	})
 }