robotframework-requests: POST request fails when both data and files are provided

When making a POST request with both data and files keyword arguments the request fails with:

$ pybot test1.txt                                                                
==============================================================================
Test1                                                                         
==============================================================================
Test One                                                              | FAIL |
ValueError: Data must not be a string.
------------------------------------------------------------------------------
Test1                                                                 | FAIL |
1 critical test, 0 passed, 1 failed
1 test total, 0 passed, 1 failed
==============================================================================

Requests Python library makes a check against data (when using files) in requests/models.py in function _encode_files:

if (not files):
    raise ValueError("Files must be provided.")
elif isinstance(data, basestring):
    raise ValueError("Data must not be a string.")

Here is test case to reproduce the error:

*Settings*

Library  RequestsLibrary
Library  OperatingSystem

*Test Case*
Test One
     Create Session  Foo  http://localhost
     ${data}=  Create Dictionary  foo=bar
     Create File  foobar  content=foobar
     ${files}=  Evaluate  open('foobar')
     Post Request  Foo  /foo  data=${data}  files=${files}

I’m using the following library versions (from pip list):

• requests (2.11.1)
• robotframework (3.0)
• robotframework-requests (0.4.5)