Implementation: Encrypt input maxPlaintextSize is NOT passed to CMM is plaintext length is known

Change: https://github.com/awslabs/aws-encryption-sdk-specification/blob/master/changes/2020-06-17_encrypt-max-plaintext-length-input/change.md
The GetEncryptionMaterials call during Encrypt MUST use the actual plaintext length (if it can be determined) for `max plaintext bound` even if a maxPlaintextSize is supplied on input.

- [ ] Python https://github.com/aws/aws-encryption-sdk-python/issues/282

Start by reading the linked change document to understand the required max plaintext bound behavior, then inspect the Python implementation issue at aws/aws-encryption-sdk-python#282. Done means the Encrypt GetEncryptionMaterials call uses the actual plaintext length when it is known, even when maxPlaintextSize is supplied.

Written by the indexing model from the issue text.