XML2RPC: add fault handling, append the faultString into error and return
xml2rpc: the value field default to stirng without any tag

Link:http://en.wikipedia.org/wiki/XML-RPC#Data_types

(copy example here)

string:
<string>Hello world!</string>
or
Hello world!

So, if not nil, it just use the raw xml as string type.
This patch add this function.
add iso8859 test case
Merge branch 'kzjeef-master' into add_faults
Adopted changes for Faults decoding by kzjeef
Merge pull request #4 from divan/add_faults

Added faults decoding support