string interpolation pads dictionaries

## Expected Behavior
No padding between key name and the value when interpolating CWL expressions that are Javascript dictionaries as per https://www.commonwl.org/v1.2/Workflow.html#String_interpolation

```
$ cwltool --quiet test.cwl
-{"baz":"zab1"}
{}
```

## Actual Behavior
A space is inserted instead

```
$ cwltool --quiet test.cwl
-{"baz": "zab1"}
{}
```

## Code
```cwl
#!/usr/bin/env cwl-runner

class: CommandLineTool
cwlVersion: v1.0
requirements:
InlineJavascriptRequirement: {}

inputs: []

baseCommand: echo

outputs: []

arguments:
- |
-$({"baz":"zab1"})
```

## Your Environment
* cwltool version: 3.0.20201113131334