Fix get dummies when uses the prefix parameter whose type is dict

[itholic]

Resolves #1469

>>> df = ks.DataFrame({'A': ['a', 'b', 'a'], 'B': ['b', 'a', 'c'], 'C': [1, 2, 3]}, columns=['A', 'B', 'C'])

>>> ks.get_dummies(df, prefix={'B': 'ohe_B', 'C': 'ohe_C'}, columns=['B', 'C'])
   A  ohe_B_a  ohe_B_b  ohe_B_c  ohe_C_1  ohe_C_2  ohe_C_3
0  a        0        1        0        1        0        0
1  b        1        0        0        0        1        0
2  a        0        0        1        0        0        1

>>> pd.get_dummies(df.to_pandas(), prefix={'B': 'ohe_B', 'C': 'ohe_C'}, columns=['B', 'C'])
   A  ohe_B_a  ohe_B_b  ohe_B_c  ohe_C_1  ohe_C_2  ohe_C_3
0  a        0        1        0        1        0        0
1  b        1        0        0        0        1        0
2  a        0        0        1        0        0        1

[codecov-io]

Codecov Report

Merging #1478 into master will decrease coverage by 1.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1478      +/-   ##
==========================================
- Coverage   93.76%   92.55%   -1.22%     
==========================================
  Files          36       36              
  Lines        8409     8411       +2     
==========================================
- Hits         7885     7785     -100     
- Misses        524      626     +102     

Impacted Files	Coverage Δ
databricks/koalas/namespace.py	86.09% <100.00%> (+0.07%)	⬆️
databricks/koalas/usage_logging/__init__.py	26.12% <0.00%> (-68.47%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	50.00% <0.00%> (-50.00%)	⬇️
databricks/koalas/__init__.py	86.53% <0.00%> (-5.77%)	⬇️
databricks/koalas/utils.py	94.14% <0.00%> (-2.44%)	⬇️
databricks/koalas/generic.py	96.65% <0.00%> (-0.38%)	⬇️
databricks/koalas/frame.py	95.33% <0.00%> (-0.25%)	⬇️

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[HyukjinKwon]

Looks pretty good otherwise