Recover from non-Ranged responses without rewinding

Not all IO objects know how to #rewind themselves. For example, Ruby
pipes (returned by `IO.pipe`) do implement #rewind, but they will throw
an error if you try to call it.

  rd, wr = IO.pipe
  wr.rewind # Errno::ESPIPE: Illegal seek

But we don't need to rewind and overwrite the IO object if we didn't get
the Ranged response we expected, we could instead wait out the content
that has already been downloaded, and start appending again once we
reached where we left off. This is what this commit does.

lib/google/apis/core/download.rb

@@ -67,12 +67,11 @@ module Google
         def execute_once(client, &block)
           request_header = header.dup
           apply_request_options(request_header)
+          download_offset = nil
 
-          check_if_rewind_needed = false
           if @offset > 0
             logger.debug { sprintf('Resuming download from offset %d', @offset) }
             request_header[RANGE_HEADER] = sprintf('bytes=%d-', @offset)
-            check_if_rewind_needed = true
           end
 
           http_res = client.get(url.to_s,
@@ -80,17 +79,24 @@ module Google
                      header: request_header,
                      follow_redirect: true) do |res, chunk|
             status = res.http_header.status_code.to_i
-            if OK_STATUS.include?(status)
-              if check_if_rewind_needed && status != 206
-                # Oh no! Requested a chunk, but received the entire content
-                # Attempt to rewind the stream
-                @download_io.rewind
-                check_if_rewind_needed = false
-              end
-              # logger.debug { sprintf('Writing chunk (%d bytes, %d total)', chunk.length, bytes_read) }
-              @download_io.write(chunk)
-              @offset += chunk.length
+            next unless OK_STATUS.include?(status)
+
+            download_offset ||= (status == 206 ? @offset : 0)
+            download_offset  += chunk.bytesize
+
+            if download_offset - chunk.bytesize == @offset
+              next_chunk = chunk
+            else
+              # Oh no! Requested a chunk, but received the entire content
+              chunk_index = @offset - (download_offset - chunk.bytesize)
+              next_chunk = chunk.byteslice(chunk_index..-1)
+              next if next_chunk.nil?
             end
+
+            # logger.debug { sprintf('Writing chunk (%d bytes, %d total)', chunk.length, bytes_read) }
+            @download_io.write(next_chunk)
+
+            @offset += next_chunk.bytesize
           end
 
           @download_io.flush