fixes if add() is successful but send() is not

[pull request 144](me-no-dev#144)

Author: serek4

src/AsyncTCP.cpp

@@ -1059,9 +1059,12 @@ size_t AsyncClient::write(const char* data) {
 
 size_t AsyncClient::write(const char* data, size_t size, uint8_t apiflags) {
     size_t will_send = add(data, size, apiflags);
-    if(!will_send || !send()) {
+    if(!will_send) {
         return 0;
     }
+    while(connected() && !send()) {
+        taskYIELD();
+    }
     return will_send;
 }