Rename Result.Headers to Result.Header.

This way, it's consistent with http.Header in the response.
diff --git a/openstack/objectstorage/v1/objects/requests.go b/openstack/objectstorage/v1/objects/requests.go
index 49dc9f5..13d94f8 100644
--- a/openstack/objectstorage/v1/objects/requests.go
+++ b/openstack/objectstorage/v1/objects/requests.go
@@ -133,7 +133,7 @@
 	body, err := ioutil.ReadAll(resp.HttpResponse.Body)
 	res.Body = body
 	res.Err = err
-	res.Headers = resp.HttpResponse.Header
+	res.Header = resp.HttpResponse.Header
 	return res
 }
 
@@ -218,7 +218,7 @@
 		MoreHeaders: h,
 		OkCodes:     []int{201},
 	})
-	res.Headers = resp.HttpResponse.Header
+	res.Header = resp.HttpResponse.Header
 	res.Err = err
 	return res
 }
@@ -274,7 +274,7 @@
 		MoreHeaders: h,
 		OkCodes:     []int{201},
 	})
-	res.Headers = resp.HttpResponse.Header
+	res.Header = resp.HttpResponse.Header
 	return res
 }
 
@@ -316,7 +316,7 @@
 		MoreHeaders: c.Provider.AuthenticatedHeaders(),
 		OkCodes:     []int{204},
 	})
-	res.Headers = resp.HttpResponse.Header
+	res.Header = resp.HttpResponse.Header
 	res.Err = err
 	return res
 }
@@ -361,7 +361,7 @@
 		MoreHeaders: c.Provider.AuthenticatedHeaders(),
 		OkCodes:     []int{200, 204},
 	})
-	res.Headers = resp.HttpResponse.Header
+	res.Header = resp.HttpResponse.Header
 	res.Err = err
 	return res
 }
@@ -418,7 +418,7 @@
 		MoreHeaders: h,
 		OkCodes:     []int{202},
 	})
-	res.Headers = resp.HttpResponse.Header
+	res.Header = resp.HttpResponse.Header
 	res.Err = err
 	return res
 }