Respect :widen when passing :lims

Previously, the limits were not widened when passing a tuple or
:round to lims (ignoring the widen argument). Fixes #3339

src/arg_desc.jl

@@ -141,6 +141,7 @@ const _arg_desc = KW(
                                 NTuple{2,Number} or Symbol. Force axis limits. Only finite values are used (you can set only the right limit with `xlims = (-Inf, 2)` for example).
                                 `:round` widens the limit to the nearest round number ie. [0.1,3.6]=>[0.0,4.0]
                                 `:symmetric` sets the limits to be symmetric around zero.
+                                Additionally pass `widen=false` to use the specified limits exactly. 
                                 """,
 :ticks                       => "Vector of numbers (set the tick values), Tuple of (tickvalues, ticklabels), or `:auto`",
 :scale                       => "Symbol. Scale of the axis: `:none`, `:ln`, `:log2`, `:log10`",

src/axes.jl

@@ -504,17 +504,14 @@ end
 const _widen_seriestypes = (:line, :path, :steppre, :stepmid, :steppost, :sticks, :scatter, :barbins, :barhist, :histogram, :scatterbins, :scatterhist, :stepbins, :stephist, :bins2d, :histogram2d, :bar, :shape, :path3d, :scatter3d)
 
 function default_should_widen(axis::Axis)
-    should_widen = false
-    if !(is_2tuple(axis[:lims]) || axis[:lims] == :round)
     for sp in axis.sps
         for series in series_list(sp)
             if series.plotattributes[:seriestype] in _widen_seriestypes
-                    should_widen = true
+                return true
             end
         end
     end
-    end
-    should_widen
+    false
 end
 
 function round_limits(amin,amax)