oauth2: ensure case sensitivity for Bearer, MAC, and Basic in auth header

Fixes #113

Change-Id: Id2ba98809a536f1dc1fab5b30c49aeedd2fe4125
Reviewed-on: https://go-review.googlesource.com/9852
Reviewed-by: Burcu Dogan <jbd@google.com>

token.go

@@ -7,6 +7,7 @@ package oauth2
 import (
 	"net/http"
 	"net/url"
+	"strings"
 	"time"
 
 	"golang.org/x/net/context"
@@ -53,6 +54,15 @@ type Token struct {
 
 // Type returns t.TokenType if non-empty, else "Bearer".
 func (t *Token) Type() string {
+	if strings.EqualFold(t.TokenType, "bearer") {
+		return "Bearer"
+	}
+	if strings.EqualFold(t.TokenType, "mac") {
+		return "MAC"
+	}
+	if strings.EqualFold(t.TokenType, "basic") {
+		return "Basic"
+	}
 	if t.TokenType != "" {
 		return t.TokenType
 	}