Allow x-amz-user-agent header

src/main/java/org/gaul/s3proxy/AwsHttpHeaders.java

@@ -35,6 +35,7 @@ final class AwsHttpHeaders {
     static final String METADATA_DIRECTIVE = "x-amz-metadata-directive";
     static final String REQUEST_ID = "x-amz-request-id";
     static final String STORAGE_CLASS = "x-amz-storage-class";
+    static final String USER_AGENT = "x-amz-user-agent";
 
     private AwsHttpHeaders() {
         throw new AssertionError("intentionally unimplemented");

src/main/java/org/gaul/s3proxy/S3ProxyHandler.java

@@ -173,7 +173,8 @@ public class S3ProxyHandler {
             AwsHttpHeaders.DATE,
             AwsHttpHeaders.DECODED_CONTENT_LENGTH,
             AwsHttpHeaders.METADATA_DIRECTIVE,
-            AwsHttpHeaders.STORAGE_CLASS
+            AwsHttpHeaders.STORAGE_CLASS,
+            AwsHttpHeaders.USER_AGENT
     );
     private static final Set<String> CANNED_ACLS = ImmutableSet.of(
             "private",