fix: Allow using As-User header with CCG Authentication (#1031)

Author: antusus

doc/authentication.md

@@ -236,18 +236,6 @@ Once you are done making calls on behalf of a managed user or app user you can s
 ```java
 api.asSelf();
 ```
-### Client Credentials Grant
-One important thing is that you can use user impersonation ony with service account API:
-```java
-BoxCCGAPIConnection api = BoxCCGAPIConnection.applicationServiceAccountConnection(
-    "client_id",
-    "client_secret",
-    "enterprise_id"
-);
-api.asUser("user_id")
-```
-
-Calling `asUser` or `asSelf` on user connection will fail with `IllegalStateException`.
 
 ## Token Exchange
 

src/main/java/com/box/sdk/BoxCCGAPIConnection.java

@@ -112,36 +112,6 @@ public boolean canRefresh() {
         return true;
     }
 
-    /**
-     * Set this API connection to make API calls on behalf of another users, impersonating them. This
-     * functionality can only be used when using service connection, otherwise it will throw
-     * IllegalStateException
-     *
-     * @param userID the ID of the user to act as.
-     * @throws IllegalStateException when called on User connection
-     */
-    @Override
-    public void asUser(String userID) {
-        if (isUserConnection()) {
-            throw new IllegalStateException("Cannot add As-User header to connection created for a user.");
-        }
-        super.asUser(userID);
-    }
-
-    /**
-     * Removes impersonation and returns connection to a service one. This undoes any previous calls to asUser().
-     *
-     * @throws IllegalStateException when called on User connection
-     * @see #asUser
-     */
-    @Override
-    public void asSelf() {
-        if (isUserConnection()) {
-            throw new IllegalStateException("Cannot remove As-User header from connection created for a user.");
-        }
-        super.asSelf();
-    }
-
     public boolean isUserConnection() {
         return subjectType.equals(USER_SUBJECT_TYPE);
     }

src/test/java/com/box/sdk/BoxCCGAPIConnectionTest.java

@@ -17,7 +17,6 @@
 import static org.hamcrest.Matchers.containsInAnyOrder;
 import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.is;
-import static org.junit.Assert.assertThrows;
 
 import com.github.tomakehurst.wiremock.junit.WireMockRule;
 import java.io.BufferedReader;
@@ -246,31 +245,6 @@ public void applicationConnectionAllowsToRemoveAsUserHeader() {
         api.refresh();
     }
 
-
-    @Test
-    public void userConnectionDoesNotAllowToSetAsUserHeader() {
-        // given
-        BoxAPIConnection api = createDefaultUserConnection();
-
-        // expect
-        assertThrows("Cannot set As-User header connection created for user.",
-            IllegalStateException.class,
-            () -> api.asUser("some_user_id")
-        );
-    }
-
-    @Test
-    public void userConnectionDoesNotAllowToRemoveAsUserHeader() {
-        // given
-        BoxAPIConnection api = createDefaultUserConnection();
-
-        // expect
-        assertThrows("Cannot remove As-User header connection created for user.",
-            IllegalStateException.class,
-            api::asSelf
-        );
-    }
-
     @Test
     public void allowsToRevokeToken() {
         // given