SpellingQueryConverter does not remove ":" char when using fielded queries

If you pass a fielded query to SpellingQueryConverter.convert, it returns a token that is prefixed with a ":" char.

Example: The query "foo:bar" is converted to ":bar"

This bug seems to have been introduced by the fix for SOLR-2556. Before this patch, SpellingQueryConverter.convert returned tokens without the leading colon char.