`description.contains:start` matches all kinds of tasks, requires quoting

taskwarrior 2.6.2 looks at `start` and thinks it has more meaning than just the text to search for:
```
$ task rc.verbose=on description.has:start export \
| jq -r '.[].description' \
| wc -l
Configuration override rc.verbose=on
Filter: ( description ~ start )
430

$ task rc.verbose=on description.has:start export \
| jq -r '.[].description' \
| grep -c start
Configuration override rc.verbose=on
Filter: ( description ~ start )
11
```
I figured out that I can use `description.contains:"start"` (with the quotes escaped through to `task`), and it works correctly, but these quotes should not be needed. It should already be obvious to the parser that because it's an argument to `.:` the RHS of `:` should not be parsed for any special meaning beyond the text itself.