collection: Prevent non-function callables from being collected

Fixes issue 331
previously to this change the collection code would issue a warning for
when ever it encountered a variable that looked like a test but wasn't a
function saying that it wouldn't collect it because it wasn't a function.
This fixes the logic so that if that warning is issued it really isn't
collected.
However previously special cases existed to support tests that were
created using functools.wraps and functools.partial. So the condition for
issuing that warning has been updated to take that in to account

Also try the old way of detecting functions just for proper integration
with mock.path in python 2.7 the get_real_func returned the unbound method

testing/python/collect.py

@@ -1013,3 +1013,27 @@ def test_collect_functools_partial(testdir):
     """)
     result = testdir.inline_run()
     result.assertoutcome(passed=6, failed=2)
+
+
+def test_dont_collect_non_function_callable(testdir):
+    """Test for issue https://github.com/pytest-dev/pytest/issues/331
+
+    In this case an INTERNALERROR occurred trying to report the failure of
+    a test like this one because py test failed to get the source lines.
+    """
+    testdir.makepyfile("""
+        class Oh(object):
+            def __call__(self):
+                pass
+
+        test_a = Oh()
+
+        def test_real():
+            pass
+    """)
+    result = testdir.runpytest('-rw')
+    result.stdout.fnmatch_lines([
+        '*collected 1 item*',
+        'WC2 *',
+        '*1 passed, 1 pytest-warnings in *',
+    ])