Fix dtype propagation

[victorjoos]

Summary

Dtype is not properly propagated to all iterators from ImageDataGenerator. This pull request fixes this

Related Issues

None

PR Overview

• [ ] This PR requires new unit tests [y/n] (make sure tests are included)
• [ ] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [x] This PR is backwards compatible [y/n]
• [ ] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

[Dref360]

Could you add a test to validate this behaviour?