Set Content-MD5 during emulated copy part

Dead store found with FindBugs.  References #56.

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

@@ -1601,6 +1601,7 @@ final class S3ProxyHandler extends AbstractHandler {
             } else {
                 Payload payload = Payloads.newInputStreamPayload(is);
                 payload.getContentMetadata().setContentLength(contentLength);
+                payload.getContentMetadata().setContentMD5(contentMD5);
 
                 MultipartPart part = blobStore.uploadMultipartPart(mpu,
                         partNumber, payload);