This fixes the issuing of a request with a doubled slash in the URL.
This causes Dropbox to issue a 301 redirect to the URL without the
`//` which causes an extra HTTP transaction.
This allows the code to deal with files > 2 GB in size. Previous to
this change a file > 2 GB would cause a JSON unmarshal error.
This is a breaking change to the API. However the Go compiler will
throw errors at the right places when compiling so should be easy to
fix up the code.
Dropbox being a REST driven service often returns useful information
in the HTTP error code. Make sure we return these in a custom Error
type if possible.
Nick Craig-Wood