[C] Keep result experience value. Fixes BUKKIT-3846

#### The Issue

When adding a custom furnace recipe with the result of an existing vanilla recipe, the experience gain is overwritten to a hard coded value of 0.1;  

#### Justification for PR

A plugin developer creating a custom recipe will unintentionally change the behaviour of vanilla recipes (less xp). 

#### PR Breakdown

Instead of setting the experience to 0.1, we get the current value the RecipesFurnace has, and use that. 

#### JIRA Ticket

BUKKIT-3846 - https://bukkit.atlassian.net/browse/BUKKIT-3846