filter() is breaking apart dot-containing '__' value part of kwarg into comma separated string?

hello, trying the examples from README, using taskwarrior 2.6.2 and current develop branch of tasklib:
```
>>> from tasklib import TaskWarrior
>>> tw = TaskWarrior()
>>> tw.tasks.pending().filter(tags__contains='testing')
[]
```
the strange thing is, look at the [slightly reformatted] trace:
```
$ sudo strace -s 999 -f -p 1752607 -e trace=execve -e status=successful
...
[pid 1753587] execve("/usr/local/bin/task", [
"task",
"rc.confirmation=no",
"rc.dependency.confirmation=no",
"rc.recurrence.confirmation=no",
"rc.json.array=off",
"rc.bulk=0",
"status:'pending'",
"tags.contains:'t,e,s,t,i,n,g'", "export"
],
0x55ba0559c100 /* 83 vars */) = 0
[pid 1753587] +++ exited with 0 +++
```
why is it breaking it apart into `t,e,s,t,i,n,g`? I have a feeling I'm doing something wrong, but can't figure out what :-) Also note that simple filters work, like
```
tw.tasks.pending().filter(description='the')
```
this translates to `description:'the'` which correctly matches tasks whose description starts with "the"

Inizia riproducendo l'esempio tw.tasks.pending().filter(tags__contains='testing') e ispeziona la gestione degli argomenti di filter() e il comando dell'attività generato mostrato nel report. La correzione è completa quando il valore viene passato come testing anziché t,e,s,t,i,n,g e il filtro corrisponde alle attività previste.

Scritto dal modello di indicizzazione a partire dal testo della issue.