Fix label & query URI encoding (#953)

* Fix label & query URI encoding

https://github.com/awslabs/aws-sdk-rust/issues/331 demonstrated that we were failing to properly encode characters for URI path components and query components in several situation. This:
- Fixes the specific bugs
- Adds proptests (run locally with 16K cases) to verify that this is the complete set.
- Adds an S3-specific protocol test that targets this issue

* Make the test a bit stronger

* Update changelog