Instagram test and Instagram node fixes

This is my work on adding the Instagram tests. A few issues might need to be sorted. I'll make comments in-line detailing these.
@knolleary I realised when developing the tests that the Instagram node's signature is not using the best phrase ever. <code>"outputType":"file"</code> has been left there for historic reasons (as in it used to download the image as a file) however now I think <code>"outputType":"buffer"</code> would be much more appropriate. Can we actually facilitate this change any time in the future please? Sorry!