use method="html", fixes display issues with anchors

I have experienced display issues when using the gomobile.mobile package in Plone 4. They were caused by anchors being modified from having an explicit end tag to a slash terminated single tag, i.e. from something like

<a name="anchor"></a>

to

<a name="anchor" />

The test test_no_modify_anchor demonstrates this issue.

Changing method="xml" to method="html" in the lxml tostring() call fixes it. I propose to change this, except of course there is a reason why method="xml" is necessary.