Stop requesting `thor-preview` in API requests

It wasn't necessary for github.com since December 2017.

features/pull_request.feature

@@ -855,7 +855,6 @@ Feature: hub pull-request
         json :html_url => "the://url", :number => 1234
       }
       post('/repos/mislav/coral/pulls/1234/requested_reviewers') {
-        halt 415 unless request.accept?('application/vnd.github.thor-preview+json')
         assert :reviewers => ["mislav", "josh", "pcorpet"]
         assert :team_reviewers => ["robots", "js"]
         status 201
@@ -877,7 +876,6 @@ Feature: hub pull-request
           :requested_teams => [{ :name => "robots" }]
       }
       post('/repos/mislav/coral/pulls/1234/requested_reviewers') {
-        halt 415 unless request.accept?('application/vnd.github.thor-preview+json')
         assert :reviewers => ["mislav", "pcorpet"]
         assert :team_reviewers => ["js"]
         status 201

github/client.go

@@ -138,7 +138,7 @@ func (client *Client) RequestReview(project *Project, prNumber int, params map[s
 		return
 	}
 
-	res, err := api.PostReview(fmt.Sprintf("repos/%s/%s/pulls/%d/requested_reviewers", project.Owner, project.Name, prNumber), params)
+	res, err := api.PostJSON(fmt.Sprintf("repos/%s/%s/pulls/%d/requested_reviewers", project.Owner, project.Name, prNumber), params)
 	if err = checkStatus(201, "requesting reviewer", res, err); err != nil {
 		return
 	}

github/http.go

@@ -279,12 +279,6 @@ func (c *simpleClient) PatchJSON(path string, payload interface{}) (*simpleRespo
 	return c.jsonRequest("PATCH", path, payload, nil)
 }
 
-func (c *simpleClient) PostReview(path string, payload interface{}) (*simpleResponse, error) {
-	return c.jsonRequest("POST", path, payload, func(req *http.Request) {
-		req.Header.Set("Accept", "application/vnd.github.thor-preview+json;charset=utf-8")
-	})
-}
-
 func (c *simpleClient) PostFile(path, filename string) (*simpleResponse, error) {
 	stat, err := os.Stat(filename)
 	if err != nil {