Bugfix: eliminate 2nd call to Close()

[BuckeyeCoder]

While researching your answer in #50 I came across a bug, and am submitting the fix which is simple.

The method qrcode.Save() is calling Close() too many times. In my test code this was causing problems as follows:

drawTo being called
Write: Called with 4096 bytes
Write: Called with 4096 bytes
Write: Called with 4096 bytes
Write: Called with 4096 bytes
Write: Called with 3509 bytes
drawTo returning
STANDARD WRITER.Write() DEFER CLOSE COMING
Close: 19893 bytes in the Buffer
QRCODE.SAVE CLOSE COMING
Close: 0 bytes in the Buffer

In this test qrcode.Save() uses the standard writer's Write() method which calls drawTo() which begins processing 4k chunks normally. When drawTo() completes and returns up the stack to standard.Writer.Write() a defer calls Close() for the first time. This causes my test code to report the size of the Buffer. Control passes up the stack back to the qrcode.Save() method which calls .Close() once again. This causes my test code to again report the size of the Buffer which is now zero.

This is obviously not desirable and potentially causes problems for some custom writers. The question becomes, "Which Close() should be eliminated?" I think the answer to that is easy, the defer in the Write() method makes much more sense to eliminate. Having the Write() and Close() together in qrcode.Save() is much more readable and proper.

This PR eliminates the superfluous deferred call to Close() in the standard writer.

@BuckeyeCoder