Changed date and datetime type conversions to use literal values in all cases (fix #126)

[laughingman7743]

TODO:

• Unit test.

[laughingman7743]

_______________ TestDefaultParameterFormatter.test_add_partition _______________
self = <tests.test_formatter.TestDefaultParameterFormatter testMethod=test_add_partition>
    def test_add_partition(self):
        expected = """
        ALTER TABLE test_table
        ADD PARTITION (dt='2017-01-01', hour=1)
        """.strip()

        actual = self.format("""
        ALTER TABLE test_table
        ADD PARTITION (dt=%(dt)s, hour=%(hour)d)
        """, {'dt': date(2017, 1, 1), 'hour': 1})
>       self.assertEqual(actual, expected)
E       AssertionError: "ALTE[19 chars]n        ADD PARTITION (dt=CAST('2017-01-01' AS DATE), hour=1)" != "ALTE[19 chars]n        ADD PARTITION (dt='2017-01-01', hour=1)"
E         ALTER TABLE test_table
E       -         ADD PARTITION (dt=CAST('2017-01-01' AS DATE), hour=1)?                           -----            ---------
E       +         ADD PARTITION (dt='2017-01-01', hour=1)

[laughingman7743]

CREATE EXTERNAL TABLE IF NOT EXISTS date_partition_table (
    a STRING
)
PARTITIONED BY (b DATE)
ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t' LINES TERMINATED BY '\n' STORED AS TEXTFILE
LOCATION 's3://BUCKET/path/to/';

A partition can be added with the following ALTER statement.
ALTER table date_partition_table ADD PARTITION (b=date '2017-01-01');

An error occurred in the following ALTER statement.
ALTER table date_partition_table ADD PARTITION (b=CAST('2017-01-01' AS DATE));
line 1:38: missing 'column' at 'partition' (service: amazonathena; status code: 400; error code: invalidrequestexception;

[laughingman7743]

In all cases, it may be better to convert to date '2020-01-01 'or timestamp '2020-01-01 00:00:00.000' using a literal value.

[codecov]

Codecov Report

Merging #127 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   93.46%   93.51%   +0.05%     
==========================================
  Files          14       14              
  Lines        1361     1357       -4     
==========================================
- Hits         1272     1269       -3     
+ Misses         89       88       -1     

Impacted Files	Coverage Δ
pyathena/formatter.py	96.15% <100.00%> (+1.03%)	⬆️

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