To support io.Bytesio

[TreeKat71]

Summary

from keras_preprocessing import image 
image.load_img(**path**, target_size)

path can be io.Byteio before, but due to the fix in #261. It doesn't work anymore.
I also read issues #293 and #294 for compatibility. Please let me know if there is any concerns.

Related Issues

#261
#293
#294

PR Overview

• [n] This PR requires new unit tests [y/n] (make sure tests are included)
• [n] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [y] This PR is backwards compatible [y/n]
• [n] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

[Dref360]

Thank you for your PR!

Please add unit tests for all cases.

[Dref360]

LGTM