Autocomplete not working after using whitespace

- [ x] I've gone though the [User Guide](https://docs.aws.amazon.com/cli/latest/userguide/cli-chap-welcome.html) and the [API reference](https://docs.aws.amazon.com/cli/latest/reference/)
- [ x] I've searched for [previous similar issues](https://github.com/aws/aws-cli/issues) and didn't find any solution

**Describe the bug**
When im using the AWS CloudShell or the terminal on my local machine autocomplete via TAB is not working after i'm using a whitespace in some value.

Example:
```
aws dynamodb scan --table-name some-table --filter-expression "Artist=:a" --[TAB] <-- Autocompletion works

aws dynamodb scan --table-name some-table --filter-expression "Artist = :a" --[TAB] <-- Autocomletion NOT working
```

**Platform/OS/Hardware/Device**
Problem occurs inside __AWS CloudShell__ and on my __Macbook__ (Terminal with zsh)

**To Reproduce (observed behavior)**
See the example above.

**Expected behavior**
Autocomplete should work if a space is used inside of an argument.

Start by reproducing the two DynamoDB scan commands in zsh or AWS CloudShell and compare autocomplete behavior with and without whitespace in the filter expression. Trace the AWS CLI completion handling for quoted arguments; done means TAB completion works for the whitespace-containing expression as well as the compact form.

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