Consistent use of IllegalStateException instead of InternalError for UnsupportedEncodingException cause

(cherry picked from commit d9b39ad)

spring-test/src/main/java/org/springframework/mock/http/MockHttpOutputMessage.java

@@ -79,7 +79,7 @@ public class MockHttpOutputMessage implements HttpOutputMessage {
 		}
 		catch (UnsupportedEncodingException ex) {
 			// should not occur
-			throw new InternalError(ex.getMessage());
+			throw new IllegalStateException(ex);
 		}
 	}