IMPALA-11325: Fix UnicodeDecodeError for shell file output

When using the --output_file commandline option for
impala-shell, the shell fails with UnicodeDecodeError
if the output contains Unicode characters.

For example, if running this command:
impala-shell -B -q "select '引'" --output_file=output.txt
This fails with:
UnicodeDecodeError : 'ascii' codec can't decode byte 0xe5 in position 0: ordinal not in range(128)

This happens due to an encode('utf-8') call happening
in OutputStream::write() on a string that is already UTF-8 encoded.
This changes the code to skip the encode('utf-8') call for Python 2.
Python 3 is using a string and still needs the encode call.

This is mostly a pragmatic fix to make the code a little bit
more functional, and there is more work to be done to have
clear contracts for the format() methods and clear points
of conversion to/from bytes.

Testing:
 - Ran shell tests with Python 2 and Python 3 on Ubuntu 18
 - Added a shell test that outputs a Unicode character
   to an output file. Without the fix, this test fails.

Change-Id: Ic40be3d530c2694465f7bd2edb0e0586ff0e1fba
Reviewed-on: http://gerrit.cloudera.org:8080/18576
Reviewed-by: Michael Smith <michael.smith@cloudera.com>
Reviewed-by: Quanlong Huang <huangquanlong@gmail.com>
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>

tests/shell/test_shell_commandline.py

@@ -1202,6 +1202,23 @@ class TestImpalaShell(ImpalaTestSuite):
       rows_from_file = [line.rstrip() for line in f]
       assert rows_from_stdout == rows_from_file
 
+  def test_output_file_utf8(self, vector, tmp_file):
+    """Test that writing UTF-8 output to a file using '--output_file' produces the
+    same output as written to stdout."""
+    # This is purely about UTF-8 output, so it doesn't need multiple rows.
+    query = "select '引'"
+    # Run the query normally and keep the stdout
+    output = run_impala_shell_cmd(vector, ['-q', query, '-B', '--output_delimiter=;'])
+    assert "Fetched 1 row(s)" in output.stderr
+    rows_from_stdout = output.stdout.strip().split('\n')
+    # Run the query with output sent to a file using '--output_file'.
+    result = run_impala_shell_cmd(vector, ['-q', query, '-B', '--output_delimiter=;',
+                                           '--output_file=%s' % tmp_file])
+    assert "Fetched 1 row(s)" in result.stderr
+    with open(tmp_file, "r") as f:
+      rows_from_file = [line.rstrip() for line in f]
+      assert rows_from_stdout == rows_from_file
+
   def test_http_socket_timeout(self, vector):
     """Test setting different http_socket_timeout_s values."""
     if (vector.get_value('strict_hs2_protocol') or