BUG: Fixed object type missmatch in SymLogNorm

Currently SymLogNorm accepts both linthresh and vmin, vmax with their
native dtype, meaning that the contents of one object cannot be updated
with another due to a mismatch of types.
Merge pull request #2428 from cpelley/symlognorm_fix

BUG: Fixed object type missmatch in SymLogNorm