Add support for Draft Pull Requests Preview API

This change implements support for a preview of a new Draft Pull
Requests API, announced at
https://developer.github.com/changes/2019-02-14-draft-pull-requests/.

• Add new media type constant.
• Use that media type in "list pull requests" and "get pull request"
  endpoints.
• Add Draft struct field to the PullRequest struct.

The "create pull request" and "update pull request" endpoints don't set
the new media type yet, but that can be dealt with in a future change.

Fixes google#1120.

github/github.go

@@ -128,6 +128,9 @@ const (
 
 	// https://developer.github.com/changes/2018-12-18-interactions-preview/
 	mediaTypeInteractionRestrictionsPreview = "application/vnd.github.sombra-preview+json"
+
+	// https://developer.github.com/changes/2019-02-14-draft-pull-requests/
+	mediaTypeDraftPreview = "application/vnd.github.shadow-cat-preview+json"
 )
 
 // A Client manages communication with the GitHub API.

github/pulls.go

@@ -32,6 +32,7 @@ type PullRequest struct {
 	MergedAt            *time.Time `json:"merged_at,omitempty"`
 	Labels              []*Label   `json:"labels,omitempty"`
 	User                *User      `json:"user,omitempty"`
+	Draft               *bool      `json:"draft,omitempty"`
 	Merged              *bool      `json:"merged,omitempty"`
 	Mergeable           *bool      `json:"mergeable,omitempty"`
 	MergeableState      *string    `json:"mergeable_state,omitempty"`
@@ -146,7 +147,7 @@ func (s *PullRequestsService) List(ctx context.Context, owner string, repo strin
 	}
 
 	// TODO: remove custom Accept header when this API fully launches.
-	acceptHeaders := []string{mediaTypeLabelDescriptionSearchPreview, mediaTypeLockReasonPreview}
+	acceptHeaders := []string{mediaTypeLabelDescriptionSearchPreview, mediaTypeLockReasonPreview, mediaTypeDraftPreview}
 	req.Header.Set("Accept", strings.Join(acceptHeaders, ", "))
 
 	var pulls []*PullRequest
@@ -169,7 +170,7 @@ func (s *PullRequestsService) Get(ctx context.Context, owner string, repo string
 	}
 
 	// TODO: remove custom Accept header when this API fully launches.
-	acceptHeaders := []string{mediaTypeLabelDescriptionSearchPreview, mediaTypeLockReasonPreview}
+	acceptHeaders := []string{mediaTypeLabelDescriptionSearchPreview, mediaTypeLockReasonPreview, mediaTypeDraftPreview}
 	req.Header.Set("Accept", strings.Join(acceptHeaders, ", "))
 
 	pull := new(PullRequest)

github/pulls_test.go

@@ -20,7 +20,7 @@ func TestPullRequestsService_List(t *testing.T) {
 	client, mux, _, teardown := setup()
 	defer teardown()
 
-	wantAcceptHeaders := []string{mediaTypeLabelDescriptionSearchPreview, mediaTypeLockReasonPreview}
+	wantAcceptHeaders := []string{mediaTypeLabelDescriptionSearchPreview, mediaTypeLockReasonPreview, mediaTypeDraftPreview}
 	mux.HandleFunc("/repos/o/r/pulls", func(w http.ResponseWriter, r *http.Request) {
 		testMethod(t, r, "GET")
 		testHeader(t, r, "Accept", strings.Join(wantAcceptHeaders, ", "))
@@ -59,7 +59,7 @@ func TestPullRequestsService_Get(t *testing.T) {
 	client, mux, _, teardown := setup()
 	defer teardown()
 
-	wantAcceptHeaders := []string{mediaTypeLabelDescriptionSearchPreview, mediaTypeLockReasonPreview}
+	wantAcceptHeaders := []string{mediaTypeLabelDescriptionSearchPreview, mediaTypeLockReasonPreview, mediaTypeDraftPreview}
 	mux.HandleFunc("/repos/o/r/pulls/1", func(w http.ResponseWriter, r *http.Request) {
 		testMethod(t, r, "GET")
 		testHeader(t, r, "Accept", strings.Join(wantAcceptHeaders, ", "))