ensure map-header shows map was saved with AC, even if imported from Cube - found by JMM

git-svn-id: svn://svn.code.sf.net/p/actiongame/code/trunk/ac@6103 e016b1c5-ed68-44aa-a2f7-31cac88cffde

source/src/worldio.cpp

@@ -374,6 +374,7 @@ void save_world(char *mname)
     copystring(hdr.mediareq, reqmpak, 128);
     stream *f = opengzfile(cgzname, "wb");
     if(!f) { conoutf("could not write map to %s", cgzname); return; }
+    strncpy(hdr.head, "ACMP", 4); // ensure map now declares itself as an AssaultCube map, even if imported as CUBE
     hdr.version = MAPVERSION;
     hdr.numents = 0;
     loopv(ents) if(ents[i].type!=NOTUSED) hdr.numents++;