jwt: allow setting a custom expiry time for JWT tokens

The current implementation of JWS/JWT in this package uses a fixed
1 hour expiry time for JWT tokens.

Some services do not accept such a long expiry time, e.g. Salesforce,
which defaults to a 5 minute expiry.
https://help.salesforce.com/HTViewHelpDoc?id=remoteaccess_oauth_jwt_flow.htm

This change adds an Expires time.Duration property to the jwt.Config
struct that, if set, will be used to calculate the jws.ClaimSet Exp property.
It allows a custom expiry to be set on a JWT token.

This change is backward compatible and will revert to previous behaviour if
the Expires property is not set.

Fixes golang/oauth2#151

Change-Id: I3159ac2a5711ef10389d83c0e290bfc7a9f54015
Reviewed-on: https://go-review.googlesource.com/14681
Reviewed-by: Burcu Dogan <jbd@google.com>

jwt/jwt.go

@@ -54,6 +54,9 @@ type Config struct {
 
 	// TokenURL is the endpoint required to complete the 2-legged JWT flow.
 	TokenURL string
+
+	// Expires optionally specifies how long the token is valid for.
+	Expires time.Duration
 }
 
 // TokenSource returns a JWT TokenSource using the configuration
@@ -95,6 +98,9 @@ func (js jwtSource) Token() (*oauth2.Token, error) {
 		// to be compatible with legacy OAuth 2.0 providers.
 		claimSet.Prn = subject
 	}
+	if t := js.conf.Expires; t > 0 {
+		claimSet.Exp = time.Now().Add(t).Unix()
+	}
 	payload, err := jws.Encode(defaultHeader, claimSet, pk)
 	if err != nil {
 		return nil, err