Inconsistent behaviour of 'accept' method

This compiles

``` scala
    val req = 
      .put("foo/bar")
      .withContent("Hello world", "text/plain")
      .accept("text/plain")

    req.send[String]()
```

But this doesn't

``` scala
    val req = client
      .put("foo/bar")
      .accept("text/plain")
      .withContent("Hello world", "text/plain")

    req.send[String]()
```

with because can't find implicit CanBuildRequest instance
