fix json -> xml request payload conversion

When creating request payload from json data, json `null` will now be converted to empty string '' not string 'None'.
Merge branch 'quiver-fix-json-to-request-payload' into develop

* quiver-fix-json-to-request-payload:
  fix json -> xml request payload conversion
Fix case conversion issue with SwapEnvironmentCNAMEs

This should be transformed to swap-environment-cnames.
This is accomplished by introducing a cache and prepopulating
the cache with specific overrides we encounter.
Add more special cases for case conversions
Merge branch 'xform-cache' into develop

* xform-cache:
  Add more special cases for case conversions
  Fix case conversion issue with SwapEnvironmentCNAMEs
Altered pagination to use ``more_results``.
Merge branch 'rds-download-logs-pagination' into develop
Fixes #231.
Updated pagination for jmespath ``result_key``s.
Pagination changes from feedback.
Updated pagination tests to add expressions.
Merge branch 'result-key-jmespath' into develop
Fixes #232.
Fixed failing tests from pagination changes.
Allow for ETS to SkipTest for HTTP status of 429
Merge branch 'ets-test-round2' into develop

* ets-test-round2:
  Allow for ETS to SkipTest for HTTP status of 429
Update JSON model with latest versions
Redirect file-like object, don't use isinstance

This fixes aws/aws-cli#544 and aws/aws-cli#654

The issue here is that the CLI uses a file like object
that implements .read() and .seek(), but doesn't actually
inherit from the file/_IOBase.
Merge branch 'follow-307-multipart' into develop

* follow-307-multipart:
  Redirect file-like object, don't use isinstance
Update pagination/extra configs for JSON models
Merge branch 'update-pagination' into develop

* update-pagination:
  Update pagination/extra configs for JSON models
Fixed assignment based on expressions.