Add support for setting "cty" header parameter

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - `Marshal` and `Unmarshal` functions.
 - `Marshaler` interface.
 - `Unmarshaler` interface.
+- Content type header parameter.
 
 ### Changed
 - Modify `Signer` signature.

header.go

@@ -4,7 +4,8 @@ package jwt
 //
 // Parameters are ordered according to the RFC 7515.
 type header struct {
-	Algorithm string `json:"alg,omitempty"`
-	KeyID     string `json:"kid,omitempty"`
-	Type      string `json:"typ,omitempty"`
+	Algorithm   string `json:"alg,omitempty"`
+	KeyID       string `json:"kid,omitempty"`
+	Type        string `json:"typ,omitempty"`
+	ContentType string `json:"cty,omitempty"`
 }

jwt.go

@@ -32,16 +32,29 @@ func (jot *JWT) Algorithm() string {
 	return jot.header().Algorithm
 }
 
+// ContentType returns the JWT's header's content type.
+func (jot *JWT) ContentType() string {
+	return jot.header().ContentType
+}
+
 // KeyID returns the JWT's header's key ID.
 func (jot *JWT) KeyID() string {
 	return jot.header().KeyID
 }
 
-// SetAlgorithm sets the algorithm a JWT uses based on its signer.
+// SetAlgorithm sets the algorithm a JWT uses to be signed.
 func (jot *JWT) SetAlgorithm(s Signer) {
 	jot.header().Algorithm = s.String()
 }
 
+// SetContentType sets the JWT's header's content type.
+//
+// This is useful if a type implements the Marshaler and the Unmarshaler
+// types in order to use JWE instead of JWS for signing and verifying.
+func (jot *JWT) SetContentType(cty string) {
+	jot.header().ContentType = cty
+}
+
 // SetKeyID sets the key ID assigned to a JWT.
 func (jot *JWT) SetKeyID(kid string) {
 	jot.header().KeyID = kid

jwt_test.go

@@ -35,6 +35,7 @@ func testJWT(t *testing.T, testCases []testCase) {
 			now := time.Now()
 			kid := fmt.Sprintf("kid %s %d", t.Name(), i)
 			typ := "JWT"
+			cty := "JWT"
 			iat := now.Unix()
 			exp := now.Add(30 * time.Minute).Unix()
 			nbf := now.Add(1 * time.Second).Unix()
@@ -60,6 +61,7 @@ func testJWT(t *testing.T, testCases []testCase) {
 			}
 			jot.SetAlgorithm(tc.signer)
 			jot.SetKeyID(kid)
+			jot.SetContentType(cty)
 
 			// 1 - Marshal.
 			payload, err := Marshal(jot)
@@ -120,6 +122,10 @@ func testJWT(t *testing.T, testCases []testCase) {
 				t.Errorf("want %s, got %s", want, got)
 			}
 
+			if want, got := cty, jot2.ContentType(); want != got {
+				t.Errorf("want %s, got %s", want, got)
+			}
+
 			if want, got := iat, jot2.IssuedAt; want != got {
 				t.Errorf("want %d, got %d", want, got)
 			}