Fix multiple zero labels when using SymLogNorm#10129

Merged

dstansby merged 2 commits intomatplotlib:masterfrom

Raab70:bug-fix/logsymnorm-zero-labels

Jan 4, 2018

Commits on Dec 30, 2017

prevent subticks from being added to zero decade
Raab70
committed

Commits on Jan 2, 2018

adding test for SymLogNorm single zero fix
Raab70
committed