Fix some SimpleTextCodec bugs

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene2878@1641854 13f79535-47bb-0310-9956-ffa450edef68
MemoryPostingsFormat.docs() should delegate to .docsAndPositions() when positions are requested in flags

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene2878@1641872 13f79535-47bb-0310-9956-ffa450edef68
score() can be called more than once, so cache frequency and sloppy frequency

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene2878@1641909 13f79535-47bb-0310-9956-ffa450edef68