Added handling of link header in githubAPI paging process

======================================================
changelog note

[#274](https://github.com/pusher/oauth2_proxy/pull/274)  Add github api pagination support (@toshi-miura ,@apratina)

======================================================

I didn't edit CHANGELOG.md.
Since # 102 was taken over and the change difference of CHANGELOG.md was large

providers/github.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"net/url"
 	"path"
+	"regexp"
 	"strconv"
 	"strings"
 
@@ -148,6 +149,7 @@ func (p *GitHubProvider) hasOrgAndTeam(accessToken string) (bool, error) {
 	}
 
 	pn := 1
+	last := 0
 	for {
 		params := url.Values{
 			"per_page": {"100"},
@@ -170,6 +172,30 @@ func (p *GitHubProvider) hasOrgAndTeam(accessToken string) (bool, error) {
 		}
 
 		body, err := ioutil.ReadAll(resp.Body)
+
+		if last == 0 {
+			// link header may not be obtained
+			// When paging is not required and all data can be retrieved with a single call
+
+			// Conditions for obtaining the link header.
+			// 1. When paging is required (Example: When the data size is 100 and the page size is 99 or less)
+			// 2. When it exceeds the paging frame (Example: When there is only 10 records but the second page is called with a page size of 100)
+
+			// link header at not last page
+			// <https://api.github.com/user/teams?page=1&per_page=100>; rel="prev", <https://api.github.com/user/teams?page=1&per_page=100>; rel="last", <https://api.github.com/user/teams?page=1&per_page=100>; rel="first"
+			// link header at last page (doesn't exist last info)
+			// <https://api.github.com/user/teams?page=3&per_page=10>; rel="prev", <https://api.github.com/user/teams?page=1&per_page=10>; rel="first"
+
+			link := resp.Header.Get("Link")
+			rep1 := regexp.MustCompile(`(?s).*\<https://api.github.com/user/teams\?page=(.)&per_page=[0-9]+\>; rel="last".*`)
+			i, converr := strconv.Atoi(rep1.ReplaceAllString(link, "$1"))
+
+			// If the last page cannot be taken from the link in the http header, the last variable remains zero
+			if converr == nil {
+				last = i
+			}
+		}
+
 		resp.Body.Close()
 		if err != nil {
 			return false, err
@@ -188,6 +214,14 @@ func (p *GitHubProvider) hasOrgAndTeam(accessToken string) (bool, error) {
 		}
 
 		teams = append(teams, tp...)
+
+		if pn == last {
+			break
+		}
+		if last == 0 {
+			break
+		}
+
 		pn++
 	}