BUG: dtype=object should stop conversion from object in frame constructo...
...r #2255

don't call maybe_convert_objects in _convert_object_array if dtype is object.

Note that it also stops other type conversions too.

It's better to add new keyword arguments at the end of the arglist, 
or you risk breaking existing code:

``` python
In [2]: def f(a,b=1):
   ...:    print b
   ...: 
   ...: f(0,'b')
   ...: 
b

In [3]: def f(a,c=2,b=1):
   ...:    print b
   ...: 
   ...: f(0,'b')
1

```
