storage: Reader uses the xml api but sends json preconditions #4470

• NewRangerReader uses the XML api to download files.
• conditionsQuery() uses the JSON API to configure preconditions
• NewRangerReader calls conditionsQuery() rather than setting the expected precondition headers.

ObjectHandle.NewReader() calls NewRangeReader:

This eventually sets req.URL.RawQuery = conditionsQuery(gen, o.conds):

conditionsQuery() appends the applicable JSON API URL parameters:

However, the readHost is not the json api, it is the xml api:

The xml API expects an x-goog-if-generation-match header. It does not recognize the JSON API's url parameters.

Thus the following will always work and never return the expected preconditon error:

client := storage.NewClient()
r, err := client.Bucket("whatever").Object("some/object").If(&storage.Conditions{GenerationMatch: 666}).NewReader(context.Background())
if err == nil {
  panic("should have failed")
}
gerr, ok := err.(*googleapi.Error)
if !ok {
  panic("should have returned a googleapi error")
}
if gerr.Code != http.PreconditionFailed {
  panic("should have return a precondition failed error")
}
// congrats! This bug no longer repros.