Fix TaurusValueComboBox does not show set value

[cmft]

When a TaurusValueComboBox widget is initialized it does not show the set value.

The setValue method does not find the given value since it saves internally as string. Fix it doing a cast.

Update by @cpascual : It is more likely related to this

[cpascual]

This fixed it for me in py3, but not in py2.(UPDATE:id does fix it) I will try to write a unit test to make sure that we fix it properly.

Also, we should check if this approach is ok for comboboxes whose values are of different types (quantities, strings, states,...)

[cpascual]

Just for future reference: a script to reproduce the problem is:

from taurus.qt.qtgui.application import TaurusApplication
from taurus.qt.qtgui.input import TaurusValueComboBox
import taurus
import sys
app = TaurusApplication(cmd_line_parser=None)
w = TaurusValueComboBox()
model = 'sys/tg_test/1/short_scalar_w'
a = taurus.Attribute(model)
a.write(2)
names = [('name0', 0), ('name1', 1), ('name2', 2), ('name3', 3)]
w.addValueNames(names)
w.setModel(model)
w.show()
sys.exit(app.exec_())

[cpascual]

I added a unit test that tests some cases more complex than those in the snippet above, and it shows that just casting to a str is not enough. In 6736f01 I propose a different approach that should pass the test. @cmft , can you please review it?

[cmft]

I agree with the changes and seems that everything work. I will merge it