Add proposal validation for method chaining.

[madhur-tandon]

When using XEITHER_OPTIONAL or XEITHER to limit valid values, the validation check doesn't happen when initialising a variable using Method Chaining.

[SylvainCorlay]

In https://github.com/jupyter-xeus/xproperty/blob/master/include/xproperty/xproperty.hpp#L182-L194

We could try replacing m_value = arg; and m_value = std::move(arg); with

m_value = owner()->template invoke_validators<T>(m_name, arg);

and

m_value = owner()->template invoke_validators<T>(m_name, std::move(arg));

respectively, and add a test.

[SylvainCorlay]

Do you want to give it a shot @madhur-tandon?

[madhur-tandon]

Was going to ask :)
I will give it a shot after completing a few things for Mark in the other project.