Presign V2: Unescape non-std queries in urls (#3549)

A client sends escaped characters in values of some query parameters in a presign url.
This commit properly unescapes queires to fix signature calculation.

cmd/signature-v2.go

@@ -111,7 +111,11 @@ func doesPresignV2SignatureMatch(r *http.Request) APIErrorCode {
 		case "Expires":
 			expires, err = url.QueryUnescape(keyval[1])
 		default:
-			filteredQueries = append(filteredQueries, query)
+			unescapedQuery, qerr := url.QueryUnescape(query)
+			if qerr == nil {
+				filteredQueries = append(filteredQueries, unescapedQuery)
+			} else {
+				err = qerr
 			}
 		}
 		// Check if the query unescaped properly.
@@ -119,6 +123,7 @@ func doesPresignV2SignatureMatch(r *http.Request) APIErrorCode {
 			errorIf(err, "Unable to unescape query values", queries)
 			return ErrInvalidQueryParams
 		}
+	}
 
 	// Invalid access key.
 	if accessKey == "" {