tiny update code style & fix typo

[Jackyele]

1. fix typo
2. tiny code style update
3. change the type of thrift_reserved_keywords to set for possible speed up

import timeit

def test_l(s=thrift_reserved_keywords):
    return 'aa' in s

def test_s(s=set(thrift_reserved_keywords)):
    return 'aa' in s

# test...
print timeit.repeat(test_l)  # 1.476s
print timeit.repeat(test_s)  # 0.143s