Fix bug in top_n function to return specified rows

tbl %.% ... %.% top_n(n) now returns n rows instead of n-1.
Add test for top_n function

To make sure it returns n rows.
Simplify top_n test
Add top_n bug fix to news
Merge branch 'master' of github.com:leondutoit/dplyr into leondutoit-master

Conflicts:
	NEWS.md
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