Fix a bug of the integer to binary converter

[t-imamichi]

Summary

Fix a bug reported at stackexchange.
This PR fixes the bug and add a unit test to cover this case.

The following code raises an error in IntegerToBinary.

from qiskit.optimization import QuadraticProgram
from qiskit.optimization.converters import IntegerToBinary
mod = QuadraticProgram()
mod.integer_var(name='x', lowerbound=0, upperbound=1)
mod.quadratic_constraint({'x': 1}, {('x', 'x'): 2}, '==', 1)
mod2 = IntegerToBinary().convert(mod)

Traceback (most recent call last):
  File "d.py", line 6, in <module>
    mod2 = IntegerToBinary().convert(mod)
  File "/Users/ima/envs/qiskit/lib/python3.7/site-packages/qiskit/optimization/converters/integer_to_binary.py", line 98, in convert
    self._substitute_int_var()
  File "/Users/ima/envs/qiskit/lib/python3.7/site-packages/qiskit/optimization/converters/integer_to_binary.py", line 201, in _substitute_int_var
    constraint.quadratic.to_dict()
  File "/Users/ima/envs/qiskit/lib/python3.7/site-packages/qiskit/optimization/converters/integer_to_binary.py", line 139, in _convert_quadratic_coefficients_dict
    x = self._src.get_variable(name_i)
  File "/Users/ima/envs/qiskit/lib/python3.7/site-packages/qiskit/optimization/problems/quadratic_program.py", line 245, in get_variable
    return self.variables[self._variables_index[i]]
KeyError: 0

Details and comments

The main reason is that there were some codes missing use_name=True of LinearExpression.to_dict and QuadraticExpression.to_dict.

I also notice that the return type does not match the definition QuadraticExpression.to_dict.
Especially, the signature of to_dict is Dict[Union[Tuple[int, int], Tuple[str, str]], float];
but, the following returns Tuple[np.int32, np.int32], float]. Because isinstance(value, int) == False with value: np.int32. This is the reason why only quadratic constraints raises this bug. I fix this issue as well.
https://github.com/Qiskit/qiskit-aqua/blob/793ffc5cfc3cfbab470ba0243fb80d42fcd28ebb/qiskit/optimization/problems/quadratic_expression.py#L171

[t-imamichi]

Thank you for your review!