Replace type of method parameter of HttpClient::SendRequest with enum

This way we advertise subset of of methods we are going to use.

BUG:24267885
Change-Id: I4dda539269b490d0f6a828342ee9182f539b225b
Reviewed-on: https://weave-review.googlesource.com/1294
Reviewed-by: Vitaly Buka <vitalybuka@google.com>
diff --git a/libweave/examples/ubuntu/curl_http_client.h b/libweave/examples/ubuntu/curl_http_client.h
index 033102d..a5090ff 100644
--- a/libweave/examples/ubuntu/curl_http_client.h
+++ b/libweave/examples/ubuntu/curl_http_client.h
@@ -24,7 +24,7 @@
  public:
   explicit CurlHttpClient(provider::TaskRunner* task_runner);
 
-  void SendRequest(const std::string& method,
+  void SendRequest(Method method,
                    const std::string& url,
                    const Headers& headers,
                    const std::string& data,