Always lstrip() keyword expression

[goodboy]

This solves the following bug.
Given and example test mod test_name_filter.py:

def test_4x4():                
    pass                       

if __name__ == '__main__':     
    import pytest              
    pytest.main(["-k not 4x4"])

Running from an IPython shell:

In [26]: run ./test_name_filter.py                                                                           
================================================= test session starts =======================================
platform linux2 -- Python 2.7.11, pytest-2.9.0, py-1.4.31, pluggy-0.3.1
rootdir: /home/tyler/pytest_bugs, inifile: 
plugins: interactive-0.1.1, ordering-0.4, instafail-0.3.0
collected 1 items 
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/main.py", line 94, in wrap_session
INTERNALERROR>     session.exitstatus = doit(config, session) or 0
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/main.py", line 124, in _main
INTERNALERROR>     config.hook.pytest_collection(session=session)
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/vendored_packages/pluggy.py", line 724, in __call__
INTERNALERROR>     return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/vendored_packages/pluggy.py", line 338, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/vendored_packages/pluggy.py", line 333, in <lambda>
INTERNALERROR>     _MultiCall(methods, kwargs, hook.spec_opts).execute()
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/vendored_packages/pluggy.py", line 596, in execute
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/main.py", line 133, in pytest_collection
INTERNALERROR>     return session.perform_collect()
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/main.py", line 567, in perform_collect
INTERNALERROR>     config=self.config, items=items)
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/vendored_packages/pluggy.py", line 724, in __call__
INTERNALERROR>     return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/vendored_packages/pluggy.py", line 338, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/vendored_packages/pluggy.py", line 333, in <lambda>
INTERNALERROR>     _MultiCall(methods, kwargs, hook.spec_opts).execute()
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/vendored_packages/pluggy.py", line 596, in execute
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/mark.py", line 78, in pytest_collection_modifyitems
INTERNALERROR>     if keywordexpr and not matchkeyword(colitem, keywordexpr):
INTERNALERROR>   File "/home/tyler/repos/pytest/_pytest/mark.py", line 161, in matchkeyword
INTERNALERROR>     return eval(keywordexpr, {}, mapping)
INTERNALERROR>   File "<string>", line 1
INTERNALERROR>     not 4x4
INTERNALERROR>           ^
INTERNALERROR> SyntaxError: unexpected EOF while parsing

============================================ no tests ran in 0.00 seconds ===================================

It will also be triggered from the console if a user mistakenly prefixes the expression with a space:

$ py.test ./test_filter_name -k ' not 4x4'

I wasn't sure if the change warranted a test but I added quick one anyway.
Also I figured this was too trivial to add to AUTHORS or CHANGELOG.

[nicoddemus]

Thanks! 😄