Default CMM should not be modifying input encryption context

The description for the default CMM's implementation of Get Encryption Materials states:

* Add the following key-value pair to the encryption context:
* ...

(https://github.com/awslabs/aws-encryption-sdk-specification/blob/master/framework/default-cmm.md#get-encryption-materials)

This value will usually come from the call to encrypt, and it is generally an anti-pattern to modify user input (which we may want to call out as an explicit tenet at the top level).

The operation should at the very least specify that a separate copy SHOULD be made instead.

Open framework/default-cmm.md and read the Get Encryption Materials section linked in the issue. Update the specification to state that a separate copy of the encryption context SHOULD be made before adding the key-value pair; the issue is done when the behavior is unambiguous in that section.

Written by the indexing model from the issue text.