BUG : errorbar does not respect `lines.linestyle` rcparam

changed default value of `fmt` from '-' -> ''

fixes issue #2366

Fix that does minimal amount of reverse compatible breakage.
I think the only scenario where this will be a problem are cases
where users have set a non-default value for `lines.linestyle`, but
do not want the lines drawn by `errorbar` to respect that.  I think that
this use case is broken because `lines.linestyle` should be respected by
plotting functions which draw lines.