Revert ClassWriter aliasing

ClassWriter prefixes columns with names that correspond to Java reserved
words with '_'. For example, 'public' becomes '_public'. Later, when
we attempt to look up the column in the Hive schema via HCatalog, the
aliased column doesn't exist (assuming we've created a Hive schema that
exactly mirrors the schema of the source DB).

This patch coarsely removes prefixed underscores. A potential
improvement would be to check that the column name indeed is a Java
reserved word.

src/java/org/apache/sqoop/mapreduce/hcat/SqoopHCatImportHelper.java

@@ -166,7 +166,7 @@ public class SqoopHCatImportHelper {
     for (Map.Entry<String, Object> entry : fieldMap.entrySet()) {
       String key = entry.getKey();
       Object val = entry.getValue();
-      String hfn = key.toLowerCase();
+      String hfn = key.toLowerCase().replaceFirst("^_", "");
       boolean skip = false;
       if (staticPartitionKeys != null && staticPartitionKeys.length > 0) {
         for (int i = 0; i < staticPartitionKeys.length; ++i) {