Support pandas Series and DataFrame as input for tf.constant

This fix tries to address the proposal raised in 2318 to support
pandas Series and DataFrame as input for tf.constant.

As was explained in 2318, Series and DataFrame has `__array__`
method, which is the standard API used for indicating that an
object is coercible into NumPy arrays.

This fix adds the support for objects with `__array__` to be
served as input for tf.constant.

This fix fixes 2318.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>