Fix 4990, allow null values in S3 object metadata

Change tests to check keys in any order instead of relying on keySets
implicit ordering.

.changes/next-release/bugfix-AmazonS3-d82f8b1.json

@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "Amazon S3",
+    "contributor": "dropofwill",
+    "description": "Fixes issue 4990 allowing null values in S3 object metadata"
+}

services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/handlers/ObjectMetadataInterceptor.java

@@ -15,8 +15,8 @@
 
 package software.amazon.awssdk.services.s3.internal.handlers;
 
+import java.util.HashMap;
 import java.util.Map;
-import java.util.stream.Collectors;
 import software.amazon.awssdk.annotations.SdkInternalApi;
 import software.amazon.awssdk.core.SdkRequest;
 import software.amazon.awssdk.core.interceptor.Context;
@@ -69,6 +69,6 @@ private CreateMultipartUploadRequest trimMetadataNames(CreateMultipartUploadRequ
 
     private Map<String, String> trimKeys(Map<String, String> map) {
         return map.entrySet().stream()
-            .collect(Collectors.toMap(e -> StringUtils.trim(e.getKey()), Map.Entry::getValue));
+            .collect(HashMap::new, (hm, entry) -> hm.put(StringUtils.trim(entry.getKey()), entry.getValue()), HashMap::putAll);
     }
 }

services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/handlers/ObjectMetadataInterceptorTest.java

@@ -20,6 +20,7 @@
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 import java.util.stream.Collectors;
@@ -37,15 +38,20 @@
 public class ObjectMetadataInterceptorTest {
     private static final ObjectMetadataInterceptor INTERCEPTOR = new ObjectMetadataInterceptor();
 
-
-
     public static List<TestCase> testCases() {
+        List<String> listWithNulls = new ArrayList<>();
+        listWithNulls.add("\tc\r\n");
+        listWithNulls.add(null);
+
+        List<String> listWithNullsTrimmed = new ArrayList<>();
+        listWithNullsTrimmed.add("c");
+        listWithNullsTrimmed.add(null);
         return asList(
             tc(asList("a", "b", "c"), asList("a", "b", "c")),
             tc(asList(" a ", "b", "c"), asList("a", "b", "c")),
             tc(asList("   a", "\tb", "\tc"), asList("a", "b", "c")),
-            tc(asList("a\n", "\tb", "\tc\r\n"), asList("a", "b", "c"))
-
+            tc(asList("a\n", "\tb", "\tc\r\n"), asList("a", "b", "c")),
+            tc(listWithNulls, listWithNullsTrimmed)
         );
     }
 
@@ -68,7 +74,7 @@ public void modifyRequest_putObject_metadataKeysAreTrimmed(TestCase tc) {
 
         PutObjectRequest modified = (PutObjectRequest) INTERCEPTOR.modifyRequest(ctx, attrs);
 
-        assertThat(modified.metadata().keySet()).containsExactlyElementsOf(tc.expectedKeys);
+        assertThat(modified.metadata().keySet()).containsOnlyOnceElementsOf(tc.expectedKeys);
     }
 
     @ParameterizedTest
@@ -90,7 +96,7 @@ public void modifyRequest_creatMultipartUpload_metadataKeysAreTrimmed(TestCase t
 
         CreateMultipartUploadRequest modified = (CreateMultipartUploadRequest) INTERCEPTOR.modifyRequest(ctx, attrs);
 
-        assertThat(modified.metadata().keySet()).containsExactlyElementsOf(tc.expectedKeys);
+        assertThat(modified.metadata().keySet()).containsOnlyOnceElementsOf(tc.expectedKeys);
     }
 
     @Test