#290, add a Show instance for Request. Things which can't be shown are shown as their type.

With this change:

    *Network.Wai> show defaultRequest
    "Request {requestMethod = \"GET\", httpVersion = HTTP/1.0, rawPathInfo = \"\", r
    awQueryString = \"\", requestHeaders = [], isSecure = False, remoteHost = 0.0.0.
    0:0, pathInfo = [], queryString = [], requestBody = <IO ByteString>, vault = <Va
    ult>, requestBodyLength = KnownLength 0, requestHeaderHost = Nothing, requestHea
    derRange = Nothing}"

Note that now determined (and evil) users can pull fields out without going through the accessors. However, that's a real pain, so people probably won't bother, and if they do having their code break seems a reasonable result.