Consume buffer when creating AWSException

At least partially fixes #537. While investigating the depths of how AWS.jl
and HTTP.jl interact, and working on several PRs to improve HTTP.jl, I
noticed a pretty simple way to fix #537 in the most common case where
the user doesn't pass in their own response_stream. Because the default
response_stream is an `IOBuffer`, we can "consume" it's contents when
an AWSException is thrown and thus when the request is retried, the buffer
will be "clean" in that the exception response body has been extracted.

src/AWSExceptions.jl

@@ -55,6 +55,7 @@ function Base.show(io::IO, e::AWSException)
 end
 
 AWSException(e::HTTP.StatusError) = AWSException(e, String(copy(e.response.body)))
+AWSException(e::HTTP.StatusError, io::IOBuffer) = AWSException(e, String(take!(io)))
 
 function AWSException(e::HTTP.StatusError, stream::IO)
     seekstart(stream)