Removing redundant calls to string::c_str()

Removing redundant calls to string::c_str().

This is mostly a mechanical change done by an internal tool.  The efficiency in
this code path (unit test) is not important at all, however I'm still doing it
in order to silence the tool and allow it to run on other parts of the code.

Thanks,
-Ivan

[Ted Mielczarek]

LGTM