Encode content before calling LWP

When LWP encodes a HASH-ref, as is done in generic_solr_request it will use the
URI module to create the www-form-urlencoded content from the HASH-ref.

The URI module will try to deduce the desired target charset from the utf8-flag
on the strings, which means that the strings sent to Solr aren't UTF-8 encoded
unless they have been either decoded, or encoded as utf8.

This is confusing, as e.g. the string "\xc6" is passed on to Solr as an UTF-8
encoded Æ passed through the add-method, but the same string will result in an
error from Solr if used with the search-method.

To fix this issue, encode all of the strings from the parameters in
generic_solr_request before passing them on to LWP. This way the charset
behaviour of generic_solr_request and _send_update is aligned.

Note: This will break applications that encode strings to UTF-8 before calling
WebService::Solr generic_solr_request, search or auto_suggest.