a=mediaclk:direct syntax

RFC 7273 says:

```
direct = "direct" [ "=" 1*DIGIT ] [SP rate]
rate   = "rate=" integer "/" integer
```

However, the tests for this only check the first part and allow any string for the rate part.
And the `makeSDP` function doesn't appear to put the `/` and denominator in the generated value.

(I've also seen `a=mediaclk:direct=0 rate=48000` in the wild.)
