Fix as_spark_type to not support "bigint".

[ueshin]

Fix as_spark_type to not support "bigint".

The string "bigint" is not recognizable by np.dtype and it causes an unexpected error:

>>> import numpy as np
>>> from databricks.koalas.typedef import as_spark_type
>>> as_spark_type(np.dtype("datetime64[ns]"))
Traceback (most recent call last):
...
TypeError: data type "bigint" not understood

Also, it doesn't work in pandas:

>>> pd.Series([1, 2, 3], dtype="bigint")
Traceback (most recent call last):
...
TypeError: data type "bigint" not understood

[codecov-io]

Codecov Report

Merging #2011 (f78c76f) into master (26e5a2d) will decrease coverage by 0.04%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #2011      +/-   ##
==========================================
- Coverage   94.58%   94.54%   -0.05%     
==========================================
  Files          50       50              
  Lines       11192    11200       +8     
==========================================
+ Hits        10586    10589       +3     
- Misses        606      611       +5     

Impacted Files	Coverage Δ
databricks/koalas/typedef/typehints.py	90.32% <44.44%> (-3.65%)	⬇️
databricks/koalas/base.py	97.20% <100.00%> (ø)

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[HyukjinKwon]

LGTM

[itholic]

LGTM

[ueshin]

Thanks! merging.