fix http parsing (#5262)

* follows #5252
* use const refs where relevant (aka stop being nasty with ram and cpu)

cores/esp8266/WString.cpp

@@ -796,3 +796,7 @@ float String::toFloat(void) const {
         return atof(buffer);
     return 0;
 }
+
+// global empty string to allow returning const String& with nothing
+
+const String emptyString;

cores/esp8266/WString.h

@@ -294,5 +294,7 @@ class StringSumHelper: public String {
         }
 };
 
+extern const String emptyString;
+
 #endif  // __cplusplus
 #endif  // String_class_h