Fix SubwordVocab

[nzw0301]

Fix #398 and use max_size.

from torchtext.vocab import SubwordVocab
from collections import Counter

counter = Counter("torch torch test".split())

v1 = SubwordVocab(counter, max_size=10)
print(v1.itos, v1.stoi)

v2 = SubwordVocab(counter, max_size=10)
print(v2.itos, v2.stoi)

This version's output

# v1
['<pad>', 't', 'c', 'e', 'h', 'o', 'r', 's', 'torch', 'test', 'est'] defaultdict(<function _default_unk_index at 0x11bc13158>,
{'<pad>': 0, 't': 1, 'c': 2, 'e': 3, 'h': 4, 'o': 5, 'r': 6, 's': 7, 'torch': 8, 'test': 9, 'est': 10})

# v2: the same to v1
['<pad>', 't', 'c', 'e', 'h', 'o', 'r', 's', 'torch', 'test', 'est'] defaultdict(<function _default_unk_index at 0x11bc13158>,
{'<pad>': 0, 't': 1, 'c': 2, 'e': 3, 'h': 4, 'o': 5, 'r': 6, 's': 7, 'torch': 8, 'test': 9, 'est': 10})

[mttk]

Thanks @nzw0301