Fix java 8 compilation (#378)

Author: TheoKanning

service/src/test/java/com/theokanning/openai/service/FileTest.java

@@ -8,8 +8,9 @@
 import org.junit.jupiter.api.TestMethodOrder;
 
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
-import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.List;
 import java.util.concurrent.TimeUnit;
 
@@ -21,7 +22,7 @@ public class FileTest {
     static String filePath = "src/test/resources/fine-tuning-data.jsonl";
 
     String token = System.getenv("OPENAI_TOKEN");
-    com.theokanning.openai.service.OpenAiService service = new OpenAiService(token);
+    OpenAiService service = new OpenAiService(token);
     static String fileId;
 
     @Test
@@ -57,7 +58,8 @@ void retrieveFile() {
     @Order(4)
     void retrieveFileContent() throws IOException {
         String fileBytesToString = service.retrieveFileContent(fileId).string();
-        assertEquals(Files.readString(Path.of(filePath)), fileBytesToString);
+        String contents = new String(Files.readAllBytes(Paths.get(filePath)), StandardCharsets.UTF_8);
+        assertEquals(contents, fileBytesToString);
     }
 
     @Test