For tcx files `HeartRateBpm` doesn't show up in the geojson feature properties

[RalucaNicola]

Parsing a tcx file I noticed that heartRates coordinate property are not added to the final geojson feature properties.

Repro:

1. Open up this glitch example: https://glitch.com/edit/#!/defiant-unleashed-ghoul
2. Click Show -> Next to code
3. Open up the console and inspect the feature in geojson -> it has a coordTimes, but not heartRates.
4. Notice that the original dataset has HeartRateBpm: https://github.com/RalucaNicola/togeojson/blob/raluca/heartRateTcx/test/data/tour_hr.tcx

[tmcw]

Fixed in #42!