BUG : errorbar does not respect lines.linestyle rcparam

[tacaswell]

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.

[tacaswell]

Had this against master, changed my mind and put it against 1.3, and now I finished that I think it should be against master instead.

[efiring]

This doesn't look right to me. First, I agree with @tacaswell that this should be a 1.4 change, not 1.3 (which might be futile anyway, if another 1.3.x never emerges). Second, the docstring needs to be changed to reflect the new behavior. To be consistent with mpl practice, I think it would be this:

• fmt=None (default) do the default, which is passing "" to plot, hence using the line rc defaults
• fmt='none' (case insensitive), don't plot the connecting lines (or markers)
• otherwise pass fmt to plot as the usual third argument.

My suspicion is that the default (a connecting line, given the usual rcParams) is rarely what users will want; but correcting that is a bigger break with the past, and I'm not sure what the best default would be.