Fix: avoid metadata mutation in fPutObject

[ThisIsOstad]

Consider the following code:

const metadata = {
  some: 'thing',
};

for (const file of files) {
  client.fPutObject(bucketName, objectName, file, metadata, callback);
}

The problem with this code is that it mutates the metadata object. After the first iteration the object contains the 'content-type' property. What follows is the wrong 'content-type' being set on the remaining files.

I solved the problem by creating a new object instead of mutating the original one.

[kaankabalak]

LGTM, thank you very much for the contribution @MoradiDavijani!