Metadata ignores all but last key

[emperor06]

Player Lollypop on Arch Linux

The documentation suggests metadata can take multiple keys (metadata [KEY...]) but when passing multiple keys, only the last one is displayed.

Example:
$ playerctl metadata artist
MyBand

$ playerctl metadata title
MySong

$ playerctl metadata artist title
MySong

[emperor06]

I think I've found where the problem is.
I've made a naive attempt at fixing it that works for me, but I'm really not familiar with glib and low-level string memory things so it may very well segfault or memleak.
Here's the attempt, just in case: emperor06@c3700b0