Fixed value for `coth(0)`

coth(0) = zoo (Complex Infinity)
For reference: http://www.wolframalpha.com/input/?i=coth%280%29
Merge pull request #7634 from sushant-hiray/coth-fix

Fixed value for `coth(0)`
Merge branch 'master' of https://github.com/sympy/sympy into lrubackport