GCS's presigned url requires blank Content-Type

BlobStoreContext context = ContextBuilder.newBuilder("google-cloud-storage")
    .credentials(GOOGLE_ACCESS_KEY, GOOGLE_SECRET_KEY)
    .buildView(BlobStoreContext.class);

BlobStore blobStore = context.getBlobStore();

Blob blob = blobStore
    .blobBuilder("3.png")
    .forSigning()
    .contentLength(20493)
    .contentType(MediaType.PNG)   // setting contentType
    .build();

HttpRequest httpRequest = context
    .getSigner()
    .signPutBlob("cs-29337eeb-27c1-4dbc-805c-f01ba1744eb1", blob, 3600);

PUT http://storage.googleapis.com/cs-29337eeb-27c1-4dbc-805c-f01ba1744eb1/3.png?Expires=1523454042&GoogleAccessId=access-s3%40storage-app-199609.iam.gserviceaccount.com&Signature=Aoam8qw0NYIDGDKBW4PpJXPklchTndaFwk4syrw0/K4c50hrDZFKG/oARWn1nSEKqRZlBuh2BBQtRSRNNH7Ol6Qhybo%2BX1hst7PxkaUue2iQwR%2B/t8p4izl%2BTGEJ99x2KdYsZb3Q7CmJvWkPlJ27KUGdVyJf/tTDGYwtE2uekXFxMu9r355N5RsBWBftW8kKnOaZBFsjO/q25bYs%2BAa/gQ01HfoQdDSPCYZ5QZST5G1vXYLuRcxYs3Hb33N3s5hq07/Nnopoai7CtrPKR1iSm766fzMaqZaSihXAQzCge5F25esML0kUkKvK%2BQBdFESD433Y1%2B33vg%2BZ9xvVlV3UlA%3D%3D HTTP/1.1

Headers:
Content-Type: image/png

<?xml version='1.0' encoding='UTF-8'?>
<Error>
<Code>SignatureDoesNotMatch</Code>
<Message>The request signature we calculated does not match the signature you provided. Check your Google secret key and signing method.</Message>
<StringToSign>PUT

image/png
1523454042
/cs-29337eeb-27c1-4dbc-805c-f01ba1744eb1/3.png</StringToSign>
</Error>

<?xml version='1.0' encoding='UTF-8'?>
<Error>
<Code>SignatureDoesNotMatch</Code>
<Message>The request signature we calculated does not match the signature you provided. Check your Google secret key and signing method.</Message>
<StringToSign>PUT

text/plain
1523454042
/cs-29337eeb-27c1-4dbc-805c-f01ba1744eb1/3.png</StringToSign>
</Error>

@Override
public HttpRequest signPutBlob(String container, Blob blob, long timeInSeconds) {
    return sign("PUT", container, blob.getMetadata().getName(), GetOptions.NONE, timestamp.get() + timeInSeconds, null);
}

private HttpRequest sign(String method, String container, String name, GetOptions options, long expires, String contentType) {
    checkNotNull(container, "container");
    checkNotNull(name, "name");
    ...
}

blob.getMetadata().getContentMetadata().getContentType();