some little improvements

[traximus]

1.fix a problem on ACWebDAVItem's createDateStr and modifyDateStr, it will not return nil now(reference from DAVKit)

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2.fix a bug on transfering the parameter overwrite in ACWebDAVCopyRequest; It didn't transfer overwrite successfully before.

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3 fix the delegate call of ACWebDAVCopyRequest and ACWebDAVMoveRequest;

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4.ACWebDAVDownloadRequest.h&.m

• add a new var to hold the reference of currentConnection, used to cancel a request if needed;
• also add a new method cancel to cancel this request

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5.ACWebDAVDeleteRequest.m - connection: didReceiveResponse:

• response.statusCode=201 OR response.statusCode=204 all shows deleted successfully.

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6.ACWebDAVClient.h

• - add new optional delegate methods

- (BOOL)needToCancelDownload

- (void)resetCancelDownload:(BOOL)cancel

- (BOOL)needToCancelUpload

- (void)resetCancelUpload:(BOOL)cancel

used to check if need to cancel a request