Rename Result.Headers to Result.Header.

This way, it's consistent with http.Header in the response.
diff --git a/openstack/objectstorage/v1/containers/requests.go b/openstack/objectstorage/v1/containers/requests.go
index 50542f4..25b68fd 100644
--- a/openstack/objectstorage/v1/containers/requests.go
+++ b/openstack/objectstorage/v1/containers/requests.go
@@ -115,7 +115,7 @@
 		MoreHeaders: h,
 		OkCodes:     []int{201, 204},
 	})
-	res.Headers = resp.HttpResponse.Header
+	res.Header = resp.HttpResponse.Header
 	res.Err = err
 	return res
 }
@@ -127,7 +127,7 @@
 		MoreHeaders: c.Provider.AuthenticatedHeaders(),
 		OkCodes:     []int{204},
 	})
-	res.Headers = resp.HttpResponse.Header
+	res.Header = resp.HttpResponse.Header
 	res.Err = err
 	return res
 }
@@ -186,7 +186,7 @@
 		MoreHeaders: h,
 		OkCodes:     []int{204},
 	})
-	res.Headers = resp.HttpResponse.Header
+	res.Header = resp.HttpResponse.Header
 	res.Err = err
 	return res
 }
@@ -200,7 +200,7 @@
 		MoreHeaders: c.Provider.AuthenticatedHeaders(),
 		OkCodes:     []int{204},
 	})
-	res.Headers = resp.HttpResponse.Header
+	res.Header = resp.HttpResponse.Header
 	res.Err = err
 	return res
 }