Better handling of parameters errors

Nobody has claimed this yet.

Ohhhh there was a missing space before `expected_status=any` in

`Patch On Session blockfilter ${path} json=${data} headers=${headers} expected_status=any`

It would be cool to have a better error message than `AttributeError: 'str' object has no attribute 'items'`

The call stack is below if it can help to have a better message:

```
Traceback (most recent call last):
File "/usr/local/lib/python3.9/site-packages/RequestsLibrary/utils.py", line 138, in decorator
return func(*args, **kwargs)
File "/usr/local/lib/python3.9/site-packages/RequestsLibrary/RequestsOnSessionKeywords.py", line 106, in patch_on_session
response = self._common_request("patch", session, url,
File "/usr/local/lib/python3.9/site-packages/RequestsLibrary/SessionKeywords.py", line 569, in _common_request
resp = method_function(
File "/usr/local/lib/python3.9/site-packages/requests/sessions.py", line 602, in patch
return self.request('PATCH', url, data=data, **kwargs)
File "/usr/local/lib/python3.9/site-packages/requests/sessions.py", line 516, in request
prep = self.prepare_request(req)
File "/usr/local/lib/python3.9/site-packages/requests/sessions.py", line 449, in prepare_request
p.prepare(
File "/usr/local/lib/python3.9/site-packages/requests/models.py", line 315, in prepare
self.prepare_headers(headers)
File "/usr/local/lib/python3.9/site-packages/requests/models.py", line 447, in prepare_headers
for header in headers.items():
```

_Originally posted by @dprevost-LMI in https://github.com/MarketSquare/robotframework-requests/issues/314#issuecomment-778517767_

Start in RequestsOnSessionKeywords.py at patch_on_session and trace the call into SessionKeywords.py::_common_request. Reproduce the malformed keyword call with the missing space before expected_status=any, then identify where the headers value reaches requests. Done means this input produces a clearer parameter error than AttributeError: 'str' object has no attribute 'items'.

Written by the indexing model from the issue text.