base reporter: Use cson for stringification when diffing objects

This produces more compact output and prevents lines that only differ due to a dangling comma in the JSON.stringify output from showing up.

Before you would get a diff like this when the ASCIIbetically last property is missing from `e.actual`:

```
    + expected - actual

     {
       "a": 123,
    +  "b": 456
    -  "b": 456,
    -  "c": 789
     }
```

With this patch you'll get:

```
    + expected - actual

     {
       a: 123
       b: 456
    -  c: 789
     }
```