CombinedChartView half first/last bar in single bar case, and fitBars defect

Currently, combined chart with single bar will cut half of first and last bar.
In bar chart, `fitBars` is not working as expected, it's default disabled, but somehow the bars are just showing correctly, while turning on, it have more space. buggy

## Problem 1: fitBars conflict with xAxis.spaceMin/Max
see https://github.com/danielgindi/Charts/issues/3186#issuecomment-357965708

## Problem 2: combined chart bars being cut off
see https://github.com/danielgindi/Charts/issues/3186#issuecomment-358183433

## Solution
For Problem 1, we can fix the conflict and update tests
For Problem 2, so far I don't have a descent solution to unify them, as all sub chart data shares the same xAxis, and to align different charts together, tweaking the x values and axisMinimum/Maximum instead of xAxis's internal min/max values or use multiple matrix and shadow xAxis is a much lower cost.