Add namespace declaration when sending stream errors through BOSH

Stream errors are sent as stanzas named <stream:error/>.  In a normal
XMPP stream, the stream prefix would be declared on the outer stream
element, but in BOSH we need to declare this prefix on the body element
in order to produce compliant XML.
Merge pull request #207 from legoscia/bosh-stream-error-namespace

Add namespace declaration when sending stream errors through BOSH