a25b407aeb
This adds support for arbitrary headers sent with each REST request, it
will allow us to test things like different xcontent-encoding (see
50_with_headers.yaml for what this looks like).
Headers are specified at the same level as `catch`, so a request would
look like:
```yaml
- do:
headers:
Content-Type: application/yaml
get:
index: test_1
type: _all
id: 1
```
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