Fix and test for ResourceRecordSet iteration when length > 100

This PR takes fixes #1647 by taking #1648 from @jameslegg and adds:

 * PEP8 style fixes
 * Test with 101 entries instead of 2
 * Fix a bug where inserting/deleting more than 100 records at a time caused failed tests

@toastdriven @jamesls @garnaat please review.