LUCENE-1299: Properly handle when IndexReader is not null and Field is in the spell checker

Files:
modified: /lucene/java/trunk/contrib/spellchecker/src/test/org/apache/lucene/search/spell/TestSpellChecker.java (try)
modified: /lucene/java/trunk/CHANGES.txt (try)
modified: /lucene/java/trunk/contrib/spellchecker/src/java/org/apache/lucene/search/spell/SpellChecker.java (try)