Fixed a typo in JWT package (go-kit#1071)

	* define JWTContextKey as a new constant

        * mark JWTTokenContextKey as a deprecated constant

        * revise corresponding error messages

auth/jwt/middleware.go

@@ -12,9 +12,14 @@ import (
 type contextKey string
 
 const (
-	// JWTContextKey holds the key used to store a JWT in the
-	// context.
-	JWTContextKey contextKey = "JWT"
+	// JWTContextKey holds the key used to store a JWT in the context.
+	JWTContextKey contextKey = "JWTToken"
+
+	// JWTTokenContextKey is an alias for JWTContextKey.
+	//
+	// Deprecated: only exists for historical compatibility.
+	// JWTContextKey would be preferable.
+	JWTTokenContextKey = JWTContextKey
 
 	// JWTClaimsContextKey holds the key used to store the JWT Claims in the
 	// context.