Remove "before" field from Response

Listing responses only ever contain a non-empty "before" field when the
"count" parameter is provided, which is only useful for the HTML
website, not really needed for API clients

Signed-off-by: Vartan Benohanian <vartanbeno@gmail.com>

reddit/wiki.go

@@ -127,7 +127,6 @@ type wikiPageRevisionListing struct {
 	Data struct {
 		Revisions []*WikiPageRevision `json:"children"`
 		After     string              `json:"after"`
-		Before    string              `json:"before"`
 	} `json:"data"`
 }
 
@@ -135,10 +134,6 @@ func (l *wikiPageRevisionListing) After() string {
 	return l.Data.After
 }
 
-func (l *wikiPageRevisionListing) Before() string {
-	return l.Data.Before
-}
-
 // WikiPageRevision is a revision of a wiki page.
 type WikiPageRevision struct {
 	ID      string     `json:"id,omitempty"`
@@ -329,7 +324,7 @@ func (s *WikiService) revisions(ctx context.Context, subreddit, page string, opt
 	}
 
 	if opts != nil {
-		idPrefix := "WikiRevision_"
+		const idPrefix = "WikiRevision_"
 		if opts.After != "" && !strings.HasPrefix(opts.After, idPrefix) {
 			opts.After = idPrefix + opts.After
 		}