When making a request to create an upload, the size cannot be changed

To reproduce:

1. Create an upload with the wrong size for the rpm

POST http://localhost:8000/api/content-sources/v1.0/repositories/uploads/

{
   chunk_size: <chunk size>,
   size: <wrong size>,
   sha256: <rpm>

} 

      2. Recreate the upload with the correct size

POST http://localhost:8000/api/content-sources/v1.0/repositories/uploads/

{
chunk_size: <same chunk size>,
size: <correct size>,
sha256: <same rpm>

}  

       3. The response from the second request will appear to have updated the size

{
  "upload_uuid": <same upload uuid from the first request,
  "size": <correct size>
} 

     4. When uploading the chunks, the upload will ultimately fail. And the upload chunk response will return the original size.

 {\"non_field_errors\":[\"End byte is greater than upload size.\"]}"}]} 

we should consider the size when making an upload entry in the upload table, and create a new entry if the size changes