add test case for request_body handler

caddytest/spec/http/requestbody/spec.hurl

@@ -0,0 +1,35 @@
+# Configure Caddy
+POST http://localhost:2019/load
+Content-Type: text/caddyfile
+```
+{
+	skip_install_trust
+	http_port     9080
+	https_port    9443
+	local_certs
+}
+localhost {
+	log
+	request_body {
+        max_size 2B
+    }
+    reverse_proxy localhost:8000 # to fake body reading
+	handle_errors 4xx {
+		respond "OK"
+	}
+}
+http://localhost:8000 {
+	respond "Failed"
+}
+```
+
+GET https://localhost:9443
+[Options]
+insecure: true
+```
+Hello
+```
+HTTP 413
+`OK`
+
+# TODO: how to test{read,write}_timeout?