Fix ExportHtml.getPadHTMLDocument()’s signature.
Since 879ae7c67d6 on Etherpad, getPadHTMLDocument()
doesn’t take the noDocType=false
argument.
This prevent Etherpad from crashing when exporting pads using Unoconv and Etherpad >= 1.6.1
Warning: this commit will prevent export from working with Etherpad <= 1.6.1.