case-insensitive URI scheme comparison

Required according to RFC 3986:

> An implementation should accept uppercase letters as equivalent to
> lowercase in scheme names

Closes #330

src/input/plugins/CurlInputPlugin.cxx

@@ -458,8 +458,8 @@ CurlInputStream::Open(const char *url, Mutex &mutex, Cond &cond)
 static InputStream *
 input_curl_open(const char *url, Mutex &mutex, Cond &cond)
 {
-	if (strncmp(url, "http://", 7) != 0 &&
-	    strncmp(url, "https://", 8) != 0)
+	if (!StringStartsWithCaseASCII(url, "http://") &&
+	    !StringStartsWithCaseASCII(url, "https://"))
 		return nullptr;
 
 	return CurlInputStream::Open(url, mutex, cond);