Issue with loading directly download image weights

[z1103246]

Due to some network issues, urlretrieve raises error for me when it tries to download model. My solution to that normally is to manually download the .h5 file and put it in the .keras/models file. However, when I did the same here for NASNet the error still persisted. I did some digging and realized that the filed downloaded is named 'NASNet_large_no_top.h5' with capitalizations, but in your nasnet.py you are passing in 'nasnet_large_no_top.h5' with all lower case to keras_utils.getfile(). I fixed the error by renaming the .h5 file accordingly. I know this is a small thing, but it is not immediately obvious and could be frustrating to deal with. Is it possible to change the filename to the appropriate lower case version?