Log request headers.

Looks like:

```
retrofit.http.RestAdapter: ---> HTTP GET https://example.com/things
retrofit.http.RestAdapter: Authorization: Basic zomglolwtfbbq
retrofit.http.RestAdapter: ---> END HTTP
```
```
retrofit.http.RestAdapter: <--- HTTP 200 https://example.com/things (257ms)
retrofit.http.RestAdapter: {"kit":"kat"}
retrofit.http.RestAdapter: <--- END HTTP
```