ReturnData is used throughout but not actually imported
I think that resultant is meant to be the contents of the combined file

It is used for the transfer and as a diff param but isn’t actually
defined anywhere before it’s used. This seemed like the least bad place
to set it.
Passing in complex_args throws ‘unsupported parameter for module’ errors

When using assemble only params (e.g. remote_src) the copy (and I’m
guessing file) modules throw an error that the param isn’t supported.
Simply removing the complex_args param fixes it for me, but I’m not sure
that’s the correct thing to do
The file module doesn’t have it’s module_args reset

I believe that this should be reset in the same way that the copy module
does
dest_contents isn’t always defined, so don’t pass it around

I think this is also a bit of a hack since it should probably be set
before being used, I’m just not sure what it should be set to.
Merge pull request #5091 from EspadaV8/5089_assemble_fix

#5089 hack-fix assemble module