Major X labels do not work for dateline

Displaying only major labels works fine for me for line graphs, as well as for the Y values on date graphs. However, when I try displaying only makor labels on the X axis, the labels disappear altogether. The example is based on the [XY date sample code](http://www.pygal.org/en/latest/documentation/types/xy.html#date).

```
from datetime import date
dateline = pygal.DateLine(x_label_rotation=25, show_minor_x_labels=False)
dateline.x_labels = [
date(2013, 1, 1),
date(2013, 7, 1),
date(2014, 1, 1),
date(2014, 7, 1),
date(2015, 1, 1),
date(2015, 7, 1)
]
dateline.x_labels_major = [
date(2013, 1, 1),
date(2015, 7, 1)
]
dateline.add("Serie", [
(date(2013, 1, 2), 213),
(date(2013, 8, 2), 281),
(date(2014, 12, 7), 198),
(date(2015, 3, 21), 120)
])
dateline.render()
```

Start with the linked XY date sample and trace how DateLine handles x_labels_major when show_minor_x_labels is false. Reproduce the example, then verify that the major X-axis dates remain visible while minor labels are hidden.

Written by the indexing model from the issue text.