test(commands): fix EOF failure with Multipart.Read

[hoenirvili]

The fix contains the changing nil with io.EOF
because acording to the Read interface implementation
An instance, (in our case Part.Read) of this general case
is that a Reader returning a non-zero number of bytes at
the end of the input stream may return either err == EOF or err == nil.
So this means that the Part.Reader chose to return
io.EOF instead of nil.

[Kubuxu]

Could you make commit message less verbose: for example: test(commands): fix EOF failure with Multipart.Read.

Also could you add parenthesis around the error logic or split it into separate if condition? It would make this code a bit more readable.

[hoenirvili]

@Kubuxu Can you restart just the travis build? I stopped for no reason.

[hoenirvili]

@Kubuxu merge?

[whyrusleeping]

This LGTM, Thanks for the fix