FIx test/sample code for signed upload url

Example/test code for Signed PUT url is wrong

https://github.com/Automattic/knox/blob/master/test/signedUrl.test.js#L78

'x-amz-acl' goes in extraHeaders argument and is not a direct argument by itself

Open test/signedUrl.test.js around line 78 and inspect the signed PUT URL example/test. Move 'x-amz-acl' into the extraHeaders argument rather than treating it as a direct argument, then run the relevant signed URL test to confirm the example uses the correct call.

Written by the indexing model from the issue text.