LibHTTP+ProtocolServer: Use CaseInsensitiveStringTraits for headers

These are supposed to be interpreted caselessly so let's just use the
case insensitive traits throughout. This means we'll understand things
like "Content-Length" even when they send "content-length" etc.

Libraries/LibHTTP/HttpResponse.cpp

@@ -28,7 +28,7 @@
 
 namespace HTTP {
 
-HttpResponse::HttpResponse(int code, HashMap<String, String>&& headers, ByteBuffer&& payload)
+HttpResponse::HttpResponse(int code, HashMap<String, String, CaseInsensitiveStringTraits>&& headers, ByteBuffer&& payload)
     : Core::NetworkResponse(move(payload))
     , m_code(code)
     , m_headers(move(headers))