MakeGenericMethod throws ArgumentNullException

Title pretty much sums it up, was crashing in a project on this line: https://github.com/paulcbetts/refit/blob/master/Refit/RequestBuilderImplementation.cs#L129

```text
An exception of type 'System.ArgumentNullException' occurred in mscorlib.dll but was not handled in user code

Additional information: Value cannot be null.
```