BUG: to_clipboard raises an error with null values in data frame
With the latest version of pandas installed on windows, running the following code will raise an error:

```
df = pd.DataFrame(['\x00'])
df.to_clipboard(index=False, excel=False)
```

Here is the traceback:

<details>

ArgumentError                             Traceback (most recent call last)

<ipython-input-7-2549db3ae4d0> in <module>
      1 df = pd.DataFrame(['\x00'])
----> 2 df.to_clipboard(index=False, excel=False)

e:\Anaconda\lib\site-packages\pandas\core\generic.py in to_clipboard(self, excel, sep, **kwargs)
   3023         from pandas.io import clipboards
   3024 
-> 3025         clipboards.to_clipboard(self, excel=excel, sep=sep, **kwargs)
   3026 
   3027     @final

e:\Anaconda\lib\site-packages\pandas\io\clipboards.py in to_clipboard(obj, excel, sep, **kwargs)
    143     else:
    144         objstr = str(obj)
--> 145     clipboard_set(objstr)

e:\Anaconda\lib\site-packages\pandas\io\clipboard\__init__.py in lazy_load_stub_copy(text)
    631     global copy, paste
    632     copy, paste = determine_clipboard()
--> 633     return copy(text)
    634 
    635 

e:\Anaconda\lib\site-packages\pandas\io\clipboard\__init__.py in copy_windows(text)
    454                     # the object must have been allocated using the
    455                     # function with the GMEM_MOVEABLE flag.
--> 456                     count = wcslen(text) + 1
    457                     handle = safeGlobalAlloc(GMEM_MOVEABLE, count * sizeof(c_wchar))
    458                     locked_handle = safeGlobalLock(handle)

e:\Anaconda\lib\site-packages\pandas\io\clipboard\__init__.py in __call__(self, *args)
    306 
    307     def __call__(self, *args):
--> 308         ret = self.f(*args)
    309         if not ret and get_errno():
    310             raise PyperclipWindowsException("Error calling " + self.f.__name__)

ArgumentError: argument 1: <class 'ValueError'>: embedded null character


</details>
