Allow for passing a string value to the getbit method

[kmasuda-aiming]

#200 setbit allow string value, but getbit not allow string value.

> require 'mock_redis'
> mr = MockRedis.new
> mr.set "key", "A"
> mr.setbit "key", "6", 1
NoMethodError: undefined method `/' for "6":String
from mock_redis-0.27.0/lib/mock_redis/string_methods.rb:96:in `getbit'

Fix above error.