Fix MKCOL response when we have an EEXIST.

Per the WebDAV spec, the correct MKCOL response for when a collection
already exists is 405.

Without this fix, `gfal-mkdir` interprets this as a file-not-found.

src/XrdHttp/XrdHttpReq.cc

@@ -943,6 +943,9 @@ void XrdHttpReq::mapXrdErrorToHttpStatus() {
       case kXR_isDirectory:
         httpStatusCode = 409; httpStatusText = "Resource is a directory";
         break;
+      case kXR_InvalidRequest:
+        httpStatusCode = 405; httpStatusText = "Method is not allowed";
+        break;
       default:
         break;
     }