Improve to handle a illegal copy request.

[ksauzz]

This is a only API compatibility issue for response.

S3 respond 400 if x-amz-copy-source is the same as object name, although CS handle it normally. This looks harmless, caused no corrupted data, and no error happens.

Error response from S3.

<Error>
  <Code>InvalidRequest</Code>
  <Message>This copy request is illegal because it is trying to copy an object to itself without changing the object's metadata, storage class, website redirect location or encryption attributes.</Message>
  <RequestId>F2F765742120B4E0</RequestId>
  <HostId>oDDrLKmzvydPJGxYGhmoOqPwajCTh63h14MkbHZCiSezLV1fk0b1udut4gQmFk3DRhjaOk9AW4w=</HostId>
</Error>"