Fix missing space when setting Prompt{,Char}

Tested with #2160. Shouldn't conflict.

Before:

```
msf > set PromptChar ~
PromptChar => ~
msf~ 
```

```
msf > set Prompt 8D
Prompt => 8D
8D> 
```

After:

```
msf > set PromptChar ~
PromptChar => ~
msf ~ 
```

```
msf > set Prompt 8D
Prompt => 8D
8D > 
```