TRUNK-6466: Restore proxies as list

Turns out we were using the list `remove()` function guarantee of only
removing a single result at a time

Author: ibacher

api/src/main/java/org/openmrs/api/context/UserContext.java

@@ -14,7 +14,7 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashSet;
-import java.util.LinkedHashSet;
+import java.util.List;
 import java.util.Locale;
 import java.util.Objects;
 import java.util.Set;
@@ -61,7 +61,7 @@ public class UserContext implements Serializable {
 	/**
 	 * User's permission proxies
 	 */
-	private final Set<String> proxies = Collections.synchronizedSet(new LinkedHashSet<>());
+	private final List<String> proxies = Collections.synchronizedList(new ArrayList<>());
 
 	/**
 	 * User's locale

api/src/test/java/org/openmrs/api/context/UserContextTest.java

@@ -22,8 +22,6 @@
 import org.springframework.beans.factory.annotation.Autowired;
 
 import static org.hamcrest.MatcherAssert.assertThat;
-import static org.hamcrest.Matchers.contains;
-import static org.hamcrest.Matchers.empty;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.nullValue;
@@ -168,4 +166,42 @@ void removeProxyPrivilege_shouldHandleNullArrayGracefully() {
 		// assert - should still have the privilege since null was passed
 		assertThat(userContext.hasPrivilege("Privilege1"), is(true));
 	}
+
+	@Test
+	void removeProxyPrivilege_shouldHandleNonExistentPrivilegeGracefully() {
+		// arrange
+		UserContext userContext = new UserContext(new TestUsernameAuthenticationScheme());
+
+		// act
+		userContext.removeProxyPrivilege("Privilege 1");
+
+		// assert - no error thrown
+	}
+	
+	@Test
+	void proxyPrivileges_shouldStackCorrectly() {
+		// arrange
+		UserContext userContext = new UserContext(new TestUsernameAuthenticationScheme());
+		
+		// act - deep nesting
+		userContext.addProxyPrivilege("Privilege1");
+		try {
+			userContext.addProxyPrivilege("Privilege1");
+			try {
+				userContext.addProxyPrivilege("Privilege1");
+				try {
+					userContext.addProxyPrivilege("Privilege1");
+				} finally {
+					userContext.removeProxyPrivilege("Privilege1");
+				}
+			} finally {
+				userContext.removeProxyPrivilege("Privilege1");
+			}
+		} finally {
+			userContext.removeProxyPrivilege("Privilege1");
+		}
+		
+		// assert
+		assertThat(userContext.hasPrivilege("Privilege1"), is(true));
+	}
 }