[FIXED DOXIA-509] Multi-markdown metadata is too greedy.

- The regex is in multi-line mode so \s will match newlines thus allowing a single empty line to be
  considered as a valid continuation line. Changing to \p{Blank} fixes the issue.
- The original test case did not validate the expected output correctly, so updated the test case
  to have more than one section (one symptom of the bug was that only the last section header and
  paragraph would avoid being mistaken for metadata continuation) and the expected output
  to match the new input correctly


git-svn-id: https://svn.apache.org/repos/asf/maven/doxia/doxia/trunk@1544070 13f79535-47bb-0310-9956-ffa450edef68