Implement HSTS fetch step

Implemented step nine of the main fetch. If current URL scheme is 'HTTP'
and current URL's host is domain and if current URL's host matched with
Known
HSTS Host Domain Name Matching results in either a superdomain match with
an asserted includeSubDomains directive or a congruent match then we
change request scheme to 'https'. This change has been made in method.rs

A test case to validate this has been added in fetch.rs. For asserting
https scheme, a https localhost was required. For this purpose I have
created a self-signed certificate and refactored fetch-context and
connector.rs to programmatically trust this certificate for running this
test case.

tests/unit/net/lib.rs

@@ -55,7 +55,7 @@ struct FetchResponseCollector {
 
 fn new_fetch_context(dc: Option<Sender<DevtoolsControlMsg>>) -> FetchContext {
     FetchContext {
-        state: HttpState::new(),
+        state: HttpState::new("certs"),
         user_agent: DEFAULT_USER_AGENT.into(),
         devtools_chan: dc,
         filemanager: FileManager::new(),