Series: Passed dummy logx parameter in _eval_nseries

This change will lose logarithmic order factors as the logx parameter
is not included in big O notation.
Added test for issue 2753

Also changed the test in test_nseries which caused failure
It now does not include logx term in big O notation.
Updated another test-case

This is identical to the one present in series/tests
Fixed issue that logx was not included in big O notation

Also reverted testcases that were changed earlier
Fixed condition where logx is None
Stylistic changes: replaced "if logx is not None" with "if logx"
Changed leadterm to handle logx parameter
Stylistic changes: removed extra line
Merge pull request #2812 from avichaldayal/2753_series

Series: Passed dummy logx parameter in _eval_nseries