Implement HTTPClient in HTML5 platform

Limitations:

 - Subject to same-origin policy
 - No persistent connection (but simulated for compatibility)
 - No blocking mode
 - No StreamPeer access
 - No chunked responses
 - Cannot disable host verification

core/io/http_client.h

@@ -131,6 +131,7 @@ public:
 	};
 
 private:
+#ifndef JAVASCRIPT_ENABLED
 	Status status;
 	IP::ResolverID resolving;
 	int conn_port;
@@ -152,14 +153,19 @@ private:
 
 	int response_num;
 	Vector<String> response_headers;
-
-	static void _bind_methods();
-	PoolStringArray _get_response_headers();
-	Dictionary _get_response_headers_as_dictionary();
 	int read_chunk_size;
 
 	Error _get_http_data(uint8_t *p_buffer, int p_bytes, int &r_received);
 
+#else
+#include "platform/javascript/http_client.h.inc"
+#endif
+
+	PoolStringArray _get_response_headers();
+	Dictionary _get_response_headers_as_dictionary();
+
+	static void _bind_methods();
+
 public:
 	//Error connect_and_get(const String& p_url,bool p_verify_host=true); //connects to a full url and perform request
 	Error connect_to_host(const String &p_host, int p_port, bool p_ssl = false, bool p_verify_host = true);