Fix bug in the GomockTestReporter

The variadic function arguments where passed as array instead of a
regular list of arguments.

This will greatly obfuscate the error messages and result in something like
```no matching expected call: []interface {}.%!v(MISSING)(%!v(MISSING))```

See http://golang.org/doc/effective_go.html#Printing for more details on how to pass variadic function arguments.