431/fix post json for token in v1.4

Author: Damian Aberbuj

lib/oauth2/client.rb

@@ -166,7 +166,11 @@ def get_token(params, access_token_opts = {}, extract_access_token = options[:ex
       opts = {:raise_errors => options[:raise_errors], :parse => params.delete(:parse)}
       headers = params.delete(:headers) || {}
       if options[:token_method] == :post
-        opts[:body] = params
+        opts[:body] = if headers['Content-Type'] == 'application/json'
+                        params.to_json
+                      else
+                        params
+                      end
         opts[:headers] = {'Content-Type' => 'application/x-www-form-urlencoded'}
       else
         opts[:params] = params