requestContentType is added as a argument to encode method

PUT and POST fail with groovy.lang.MissingMethodException if requestContentType is other than URLENC.

Sample stacktrace when requestContentType set to XML:
groovy.lang.MissingMethodException: No signature of method: groovyx.net.http.EncoderRegistry.encodeXML() is applicable for argument types: (java.util.LinkedHashMap) values: [[status:HTTPBuilder unit test was run on Sun May 25 12:37:24 CEST 2014, ...]]