BUG: Fix bug in type conversion of arrays; add array and struct tests

[jasonqng]

As per @tswast's requests in #22 and #23, tests added to check that arrays and structs are successfully returned and named properly from a query.

While running tests, identified a bug with the type conversion in read_gbq (see below). Updated it so it only performs type conversion with the added condition that the returned field mode IS NOT repeated.

To re-produce type conversion error with array:

>>> import gbq
>>> query = """WITH t as (
...         SELECT "a" letter, 1 num
...         UNION ALL
...         SELECT "b" letter, 2 num
...         UNION ALL
...         SELECT "a" letter, 3 num)
...
...         select letter, array_agg(num order by num ASC) num_array
...         from t
...         group by letter"""
>>> df = gbq.read_gbq(query, project_id='XXXXX',dialect='standard',verbose=False)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "gbq.py", line 877, in read_gbq
    final_df[field['name']].astype(type_map[field['type'].upper()])
  File "/Users/jasonng/anaconda2/lib/python2.7/site-packages/pandas/util/_decorators.py", line 118, in wrapper
    return func(*args, **kwargs)
  File "/Users/jasonng/anaconda2/lib/python2.7/site-packages/pandas/core/generic.py", line 4004, in astype
    **kwargs)
  File "/Users/jasonng/anaconda2/lib/python2.7/site-packages/pandas/core/internals.py", line 3462, in astype
    return self.apply('astype', dtype=dtype, **kwargs)
  File "/Users/jasonng/anaconda2/lib/python2.7/site-packages/pandas/core/internals.py", line 3329, in apply
    applied = getattr(b, f)(**kwargs)
  File "/Users/jasonng/anaconda2/lib/python2.7/site-packages/pandas/core/internals.py", line 544, in astype
    **kwargs)
  File "/Users/jasonng/anaconda2/lib/python2.7/site-packages/pandas/core/internals.py", line 625, in _astype
    values = astype_nansafe(values.ravel(), dtype, copy=True)
  File "/Users/jasonng/anaconda2/lib/python2.7/site-packages/pandas/core/dtypes/cast.py", line 692, in astype_nansafe
    return lib.astype_intsafe(arr.ravel(), dtype).reshape(arr.shape)
  File "pandas/_libs/lib.pyx", line 854, in pandas._libs.lib.astype_intsafe
  File "pandas/_libs/src/util.pxd", line 91, in util.set_value_at_unsafe
ValueError: setting an array element with a sequence.

[codecov-io]

Codecov Report

Merging #101 into master will decrease coverage by 44.57%.
The diff coverage is 25%.

@@             Coverage Diff             @@
##           master     #101       +/-   ##
===========================================
- Coverage    73.5%   28.93%   -44.58%     
===========================================
  Files           4        4               
  Lines        1491     1507       +16     
===========================================
- Hits         1096      436      -660     
- Misses        395     1071      +676

Impacted Files	Coverage Δ
pandas_gbq/gbq.py	20.55% <ø> (-55.94%)	⬇️
pandas_gbq/tests/test_gbq.py	27.92% <25%> (-54.42%)	⬇️

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[tswast]

Thanks!

[tswast]

@jasonqng Could you add this to the 0.3.0 entry in the changelog? https://github.com/pydata/pandas-gbq/blob/master/docs/source/changelog.rst

[jasonqng]

@tswast Added, feel free to tweak.