Make sure path components get escaped

The derived client for an API containing `Capture` has a bug: it does not
escape characters, so that if the string is "a/b", the URL becomes
`".../a/b/..."` instead of `".../a%2Fb/..."`, causing the corresponding servant
server to return a 404.

This relies on https://github.com/fizruk/http-api-data/pull/47

servant-client/src/Servant/Common/Req.hs

@@ -27,6 +27,7 @@ import Control.Monad.Base (MonadBase (..))
 import Control.Monad.IO.Class ()
 import Control.Monad.Reader
 import Control.Monad.Trans.Control (MonadBaseControl (..))
+import qualified Data.ByteString.Builder as BS
 import Data.ByteString.Lazy hiding (pack, filter, map, null, elem, any)
 import Data.String
 import Data.String.Conversions (cs)
@@ -86,7 +87,7 @@ instance Eq ServantError where
 instance Exception ServantError
 
 data Req = Req
-  { reqPath   :: String
+  { reqPath   :: BS.Builder
   , qs        :: QueryText
   , reqBody   :: Maybe (RequestBody, MediaType)
   , reqAccept :: [MediaType]
@@ -98,7 +99,7 @@ defReq = Req "" [] Nothing [] []
 
 appendToPath :: String -> Req -> Req
 appendToPath p req =
-  req { reqPath = reqPath req ++ "/" ++ p }
+  req { reqPath = reqPath req <> "/" <> toEncodedUrlPiece p }
 
 appendToQueryString :: Text       -- ^ param name
                     -> Maybe Text -- ^ param value
@@ -151,8 +152,9 @@ reqToRequest req (BaseUrl reqScheme reqHost reqPort path) =
                                          , uriRegName = reqHost
                                          , uriPort = ":" ++ show reqPort
                                          }
-                             , uriPath = path ++ reqPath req
+                             , uriPath = fullPath
                              }
+        fullPath = path ++ cs (BS.toLazyByteString (reqPath req))
 
         setrqb r = case reqBody req of
                      Nothing -> r