Map field missing parts of values into respose

[Maksim-Burtsev]

1. First example i found in the tests (in current state of master 1 test test_fetchrow_with_empties didnt pass):

expected: {'hello': {'inner': "world {'"}}
received: {'hello': {'inner': 'world {'}} 

' from world {' is missing.

2. Second one is more interesting.
   Lest create table with nested Map and one them have value with Array type:

CREATE TABLE default.map_map_map
(
    `data` Map(String, Map(String, Array(String)))
)
ENGINE = MergeTree
ORDER BY data
SETTINGS index_granularity = 8192

INSERT INTO map_map_map VALUES ({'key1': {'key2': ['123', '456']}})

Expected result from SELECT query:

SELECT *
FROM map_map_map

Query id: 6a946fc5-e9c4-4a8d-96b8-727df58f0a6c

┌─data────────────────────────────┐
│ {'key1':{'key2':['123','456']}} │
└─────────────────────────────────┘

Real result:

In [1]: dict(await ch_client.fetchrow("select * from map_map_map"))
Out[1]: {'data': {'key1': {'key2': []}}}

So values of key2 are dissapeared.
And i can tell you why.

Because array type has slice [1:-1] there:

    def p_type(self, string: str) -> list:
        return [
            self.type.p_type(self.decode(val.encode()))
            for val in self.seq_parser(string[1:-1])
        ]

and its meant for string argument (which actually have str) type to remove quoutes of it, but in this case after json.loads(string) (323 line in types.py) in this method was passed list and slice just removed first and last items :)

And its confirmed with list of 3+ elements:

INSERT INTO map_map_map VALUES ({'key1': {'key2': ['123', '456', '789']}})

In [1]: dict(await ch_client.fetchrow("select * from map_map_map order by data desc"))
Out[1]: {'data': {'key1': {'key2': ['456']}}}

So right now MapType not working properly and need some fixes.

[Maksim-Burtsev]

This PR closes this issue — #108

1. No more missing quotes:

In [1]: dict(await ch_client.fetchrow("select map_map from all_types"))
Out[1]: {'map_map': {'hello': {'inner': "world {'"}}}
                                               ~~~

2. No more missing values from Array:

In [2]: dict(await ch_client.fetchrow("select * from map_map_map"))
Out[2]: {'data': {'key1': {'key2': ['123', '456']}}}