Fix bug in latex-printing Equivalent objects

Latex-printing an Equivalent object would only print out two
arguments, even if there are more.
Merge pull request #7601 from clsn/printfix

Fix bug in latex-printing Equivalent objects
Fix for series expansions in products of symbols with assumptions

For example:
http://nbviewer.ipython.org/gist/jrjohansson/bf7862198171b16d0ff9