Fix setConfigXML according to the new docs

[mconf-daileon]

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Author Name: Leonardo Daronco (Leonardo Daronco) Original Redmine Issue: 1638, http://dev.mconf.org/redmine/issues/1638 Original Date: 2015-05-27

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The documentation for @setConfigXML@ was updated recently: http://docs.bigbluebutton.org/dev/api.html#setconfigxml

The API call works by passing the data in the body of the POST request, but as long as some other requirements are met (e.g. the content type header has to be the correct type). We should fix this API call in the gem to pass the data only in the body of the request, removing it from the URL.

Note: setting a different content-type also influences how the POST requests to the API call @create@ will behave. The content type being used right now (https://github.com/mconf/bigbluebutton-api-ruby/blob/0e17a1f68380966539eb4eccffcd106a255b9275/lib/bigbluebutton_api.rb#L711) works for @create@ requests, so it should not be changed. It should be changed only for @setConfigXML@.