r=chofmann. adding content-type to default response headers for testserver

netwerk/testserver/docs/generic.res

@@ -1,2 +1,3 @@
 HTTP/1.1 200 OK
 Server: HTTP Test Server/1.1
+Content-Type: text/html

tools/testserver/docs/generic.res

@@ -1,2 +1,3 @@
 HTTP/1.1 200 OK
 Server: HTTP Test Server/1.1
+Content-Type: text/html