include collection and id in postparams

this changes the JSON tags to include both the ID and Collection
in PostParams. Both will be omitted when empty to maintain any unknown
backwards compatibility.

post.go

@@ -38,7 +38,7 @@ type (
 	// PostParams holds values for creating or updating a post.
 	PostParams struct {
 		// Parameters only for updating
-		ID    string `json:"-"`
+		ID    string `json:"id,omitempty"`
 		Token string `json:"token,omitempty"`
 
 		// Parameters for creating or updating
@@ -55,7 +55,7 @@ type (
 		Crosspost []map[string]string `json:"crosspost,omitempty"`
 
 		// Parameters for collection posts
-		Collection string `json:"-"`
+		Collection string `json:"collection,omitempty"`
 	}
 
 	// PinnedPostParams holds values for pinning a post