Fix tooltip when option is set to 'always'

[jespirit]

Fix typo with variable name. Just added _ underscore.

Changed this.alwaysShowTooltip to this._alwaysShowTooltip

https://github.com/seiyria/bootstrap-slider/blob/88dfe2aa20bb49f524b681062e6ea15d26402c41/src/js/bootstrap-slider.js#L809-L812

https://github.com/seiyria/bootstrap-slider/blob/88dfe2aa20bb49f524b681062e6ea15d26402c41/src/js/bootstrap-slider.js#L1164-L1171

Added a unit test.

Also there was a bug when options.tooltip was set to 'always' and the options.ticks_tooltip was set to true.

Pull Requests

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• [x] unit tests (we use Jasmine 2.x.x)
• [ ] JSFiddle (or an equivalent such as CodePen, Plunker, etc) or screenshot/GIF with new feature or bug-fix
• [ ] Link to original Github issue (if this is a bug-fix)
• [ ] Passes CI-server checks (runs automated tests, JS source-code linting, etc..). To run these on your local machine, type grunt test in your Terminal within the bootstrap-slider repository directory