Fix XHPAST handling of multiple-line tokens

Summary: The code emitted '<span="s">', i.e. no "class" part.

Test Plan: Created a paste with the loadAllWhere() snippet, it now highlights
correctly and the markup is non-silly.

Reviewers: codeblock, jungejason, tuomaspelkonen, aran

Reviewed By: codeblock

CC: aran, codeblock

Differential Revision: 828