Saving a metadata with non US-ASCII characters it gives an error about faulty signature

**Describe the bug**

This bug from the SDK seems to exist in the Toolkit as well:
https://github.com/aws/aws-sdk-net/issues/1587

So when trying to save a metadata with non US-ASCII characters it gives an error about faulty signature

**To Reproduce**

1. Start uploading a item
2. Add a metadata value containing a non US-ASCII character (for example £)
3. Press OK
4. Enjoy error

**Expected behavior**

For it to be saved, but encoded

BONUS:
Display the un-encoded value when retriving the properties again, let the encoding be underneath the hood

**Screenshots**

**Computer (please complete the following information):**

- Windows Version: Win 10
- Visual Studio Version: 2019
- AWS Toolkit for Visual Studio Version: 1.17.1.0

**Additional context**

Reproduce the issue in AWS Toolkit for Visual Studio by uploading an item with metadata containing a non-US-ASCII character such as £, then compare the behavior with the linked AWS SDK issue. Trace the metadata save and retrieval flow; done means the value saves without a faulty-signature error and is displayed unencoded when properties are retrieved.

Written by the indexing model from the issue text.