sasl: prefer PLAIN mechanism over LOGIN

SASL PLAIN is a standard, LOGIN only a draft. The LOGIN draft says
PLAIN should be used instead if available.

lib/curl_sasl.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 2012 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 2012 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -361,15 +361,6 @@ CURLcode Curl_sasl_start(struct SASL *sasl, struct connectdata *conn,
                                                        conn->oauth_bearer,
                                                        &resp, &len);
     }
-    else if(enabledmechs & SASL_MECH_LOGIN) {
-      mech = SASL_MECH_STRING_LOGIN;
-      state1 = SASL_LOGIN;
-      state2 = SASL_LOGIN_PASSWD;
-      sasl->authused = SASL_MECH_LOGIN;
-
-      if(force_ir || data->set.sasl_ir)
-        result = Curl_auth_create_login_message(data, conn->user, &resp, &len);
-    }
     else if(enabledmechs & SASL_MECH_PLAIN) {
       mech = SASL_MECH_STRING_PLAIN;
       state1 = SASL_PLAIN;
@@ -379,6 +370,15 @@ CURLcode Curl_sasl_start(struct SASL *sasl, struct connectdata *conn,
         result = Curl_auth_create_plain_message(data, conn->user, conn->passwd,
                                                 &resp, &len);
     }
+    else if(enabledmechs & SASL_MECH_LOGIN) {
+      mech = SASL_MECH_STRING_LOGIN;
+      state1 = SASL_LOGIN;
+      state2 = SASL_LOGIN_PASSWD;
+      sasl->authused = SASL_MECH_LOGIN;
+
+      if(force_ir || data->set.sasl_ir)
+        result = Curl_auth_create_login_message(data, conn->user, &resp, &len);
+    }
   }
 
   if(!result && mech) {