Allow submit object to be assigned non-strings.  #7065
Allow submit object to be assigned non-strings.  #7065

Currently, Submit objects must be assigned strings, as in:

```
sub["foo"] = "1"
```

We now convert to string when this can be done cleanly. Example:

```
sub["foo"] = 1
```

Further, we special case this for assigning ClassAds and Expressions:

```
sub["MY.foo"] = classad.ExprTree("A < B")
```

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