Quote schema in accept header

HTTP 1.1 does not permit a slash inside an accept extension, unless it is inside a quoted string. The Accept header used by Prometheus is incorrect.

From [RFC 7230](http://tools.ietf.org/html/rfc7230):

```
     token          = 1*tchar

     tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
                    / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
                    / DIGIT / ALPHA
                    ; any VCHAR, except delimiters
```

From [RFC rfc7231](http://tools.ietf.org/html/rfc7231):

```
    accept-ext = OWS ";" OWS token [ "=" ( token / quoted-string ) ]
```