Read API match Literals expects bare strings instead of N3

**Description**
Read API match parameters are described to be n3 but for literals only bare string notation works.

**Steps to reproduce the issue:**
1. Query with `obj="value"` doesn't work
2. Query with `obj=value` works

**Output of `cayley version` or commit hash:**

```
Cayley version: v0.7.x-dev
Git commit hash: b9dabbed3f75
```

**Environment details:**

Backend database: `memory`