proxy - fix for buggy charset detection

When querying the following WFS service http://geo.somme.fr/arcgis/services/ENS_CANTON_COLLEGE/MapServer/WFSServer, the response content-type is ```text/xml``` without charset.

IE requires that the charset is set, so the geOrchestra proxy tries to guess it 1) from the response XML 2) from the accept-charset request header.

In the current case, the response XML does not contain charset information, and there is not accept-charset request header => the local "default" is used, which happened to be ISO-8859-1 (in the calculateDefaultCharset method)

The current patch fixes this behavior, by returning the configured global default charset, which is UTF-8.